README: formatting, links, updates

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

README.md

@@ -10,31 +10,29 @@ While the traditional `mtree` cli utility is primarily on BSDs (FreeBSD,
 openBSD, etc), even broader support for the `mtree` specification format is
 provided with libarchive ([libarchive-formats(5)][libarchive-formats(5)]).
 
-There is also an [mtree port for Linux][archiecobbs/mtree-port] though it is
+There is also an [mtree port for Linux][archiecobbs/mtree-port] though it is not widely packaged for Linux distributions.
-not widely packaged for Linux distributions.
+
+There was a [Google Summer of Code project to create a portable library and parser for mtree](https://wiki.freebsd.org/SummerOfCode2015/mtreeParsingLibrary).
+It is available at [github.com/mratajsky/libmtree](https://github.com/mratajsky/libmtree) and a [talk on it](https://papers.freebsd.org/2016/asiabsdcon/ratajsky-mtree-parsing/).
 
 ## Format
 
-The format of hierarchy specification is consistent with the `# mtree v2.0`
+The BSD mtree specification is published in [mtree(5)][mtree(5)].
-format.  Both the BSD `mtree` and libarchive ought to be interoperable with it
-with only one definite caveat.  On Linux, extended attributes (`xattr`) on
-files are often a critical aspect of the file, holding ACLs, capabilities, etc.
-While FreeBSD filesystem do support `extattr`, this feature has not made its
-way into their `mtree`.
 
-This implementation of mtree supports a few non-upstream "keyword"s, such as:
+The format of hierarchy specification is consistent with the `# mtree v2.0` format.
-`xattr` and `tar_time`. If you include these keywords, the FreeBSD `mtree`
+Both the BSD `mtree` and libarchive ought to be interoperable with it with only one definite caveat.
-will fail, as they are unknown keywords to that implementation.
+On Linux, extended attributes (`xattr`) on files are often a critical aspect of the file, holding ACLs, capabilities, etc.
+While FreeBSD filesystem do support `extattr`, this feature has not made its way into their `mtree`.
 
-To have `go-mtree` produce specifications that will be
+This implementation of mtree supports a few non-upstream "keyword"s, such as: `xattr` and `tar_time`.
-strictly compatible with the BSD `mtree`, use the `-bsd-keywords` flag when
+If you include these keywords, the FreeBSD `mtree` will fail, as they are unknown keywords to that implementation.
-creating a manifest. This will make sure that only the keywords supported by
+
-BSD `mtree` are used in the program.
+To have `go-mtree` produce specifications that will be strictly compatible with the BSD `mtree`, use the `-bsd-keywords` flag when creating a manifest.
+This will make sure that only the keywords supported by BSD `mtree` are used in the program.
 
 ### Typical form
 
-With the standard keywords, plus say `sha256digest`, the hierarchy
+With the standard keywords, plus say `sha256digest`, the hierarchy specification looks like:
-specification looks like:
 
 ```mtree
 # .
@@ -46,9 +44,9 @@ specification looks like:
 [...]
 ```
 
-See the directory presently in, and the files present. Along with each
+See the directory presently in, and the files present.
-path, is provided the keywords and the unique values for each path. Any common
+Along with each path, is provided the keywords and the unique values for each path.
-keyword and values are established in the `/set` command.
+Any common keyword and values are established in the `/set` command.
 
 ### Extended attributes form
 
@@ -62,16 +60,13 @@ keyword and values are established in the `/set` command.
 [...]
 ```
 
-See the keyword prefixed with `xattr.` followed by the extended attribute's
+See the keyword prefixed with `xattr.` followed by the extended attribute's namespace and keyword.
-namespace and keyword. This setup is consistent for use with Linux extended
+This setup is consistent for use with Linux extended attributes as well as FreeBSD extended attributes.
-attributes as well as FreeBSD extended attributes.
 
-Since extended attributes are an unordered hashmap, this approach allows for
+Since extended attributes are an unordered hashmap, this approach allows for checking each `<namespace>.<key>` individually.
-checking each `<namespace>.<key>` individually.
 
-The value is the [base64 encoded][base64] of the value of the particular
+The value is the [base64 encoded][base64] of the value of the particular extended attribute.
-extended attribute. Since the values themselves could be raw bytes, this
+Since the values themselves could be raw bytes, this approach avoids issues with encoding.
-approach avoids issues with encoding.
 
 ### Tar form
 
@@ -88,21 +83,15 @@ samedir type=dir mode=0775 tar_time=1468000972.000000000
 [...]
 ```
 
-While `go-mtree` serves mainly as a library for upstream `mtree` support,
+While `go-mtree` serves mainly as a library for upstream `mtree` support, `go-mtree` is also compatible with [tar archives][tar] (which is not an upstream feature).
-`go-mtree` is also compatible with [tar archives][tar] (which is not an upstream feature).
 This means that we can now create and validate a manifest by specifying a tar file.
-More interestingly, this also means that we can create a manifest from an archive, and then
+More interestingly, this also means that we can create a manifest from an archive, and then validate this manifest against a filesystem hierarchy that's on disk, and vice versa.
-validate this manifest against a filesystem hierarchy that's on disk, and vice versa.
 
-Notice that for the output of creating a validation manifest from a tar file, the default behavior
+Notice that for the output of creating a validation manifest from a tar file, the default behavior for evaluating a notion of time is to use the `tar_time` keyword.
-for evaluating a notion of time is to use the `tar_time` keyword. In the
+In the "filesystem hierarchy" format of mtree, `time` is being evaluated with nanosecond precision.
-"filesystem hierarchy" format of mtree, `time` is being evaluated with
+However, GNU tar truncates a file's modification time to 1-second precision.
-nanosecond precision. However, GNU tar truncates a file's modification time
+That is, if a file's full modification time is 123456789.123456789, the "tar time" equivalent would be 123456789.000000000.
-to 1-second precision. That is, if a file's full modification time is
+This way, if you validate a manifest created using a tar file against an actual root directory, there will be no complaints from `go-mtree` so long as the 1-second precision time of a file in the root directory is the same.
-123456789.123456789, the "tar time" equivalent would be 123456789.000000000.
-This way, if you validate a manifest created using a tar file against an
-actual root directory, there will be no complaints from `go-mtree` so long as the
-1-second precision time of a file in the root directory is the same.
 
 ## Usage
 
@@ -114,31 +103,31 @@ To use the command line tool, first [build it](#Building), then the following.
 
 This will also include the sha512 digest of the files.
 
-```bash
+```shell
 gomtree -c -K sha512digest -p . > /tmp/root.mtree
 ```
 
 With a tar file:
 
-```bash
+```shell
 gomtree -c -K sha512digest -T sometarfile.tar > /tmp/tar.mtree
 ```
 
 ### Validate a manifest
 
-```bash
+```shell
 gomtree -p . -f /tmp/root.mtree
 ```
 
 With a tar file:
 
-```bash
+```shell
 gomtree -T sometarfile.tar -f /tmp/root.mtree
 ```
 
 ### See the supported keywords
 
-```bash
+```shell
 gomtree -list-keywords
 Available keywords:
  uname
@@ -173,37 +162,46 @@ Available keywords:
 
 Either:
 
-```bash
+```shell
-go get github.com/vbatts/go-mtree/cmd/gomtree
+go install github.com/vbatts/go-mtree/cmd/gomtree@latest
 ```
 
 or
 
-```bash
+```shell
 git clone git://github.com/vbatts/go-mtree.git $GOPATH/src/github.com/vbatts/go-mtree
 cd $GOPATH/src/github.com/vbatts/go-mtree
 go build ./cmd/gomtree
 ```
 
+### Build for many OS/Arch
+
+```shell
+make build.arches
+```
+
 ## Testing
 
 On Linux:
 
-```bash
+```shell
 cd $GOPATH/src/github.com/vbatts/go-mtree
 make
 ```
 
 On FreeBSD:
 
-```bash
+```shell
 cd $GOPATH/src/github.com/vbatts/go-mtree
 gmake
 ```
 
+## Related tools
+
+[mtree(5)]: https://man.freebsd.org/cgi/man.cgi?query=mtree&sektion=5&format=html
 [mtree(8)]: https://www.freebsd.org/cgi/man.cgi?mtree(8)
 [libarchive-formats(5)]: https://www.freebsd.org/cgi/man.cgi?query=libarchive-formats&sektion=5&n=1
 [archiecobbs/mtree-port]: https://github.com/archiecobbs/mtree-port
-[godoc]: https://godoc.org/github.com/vbatts/go-mtree
+[godoc]: https://pkg.go.dev/github.com/vbatts/go-mtree
 [tar]: http://man7.org/linux/man-pages/man1/tar.1.html
 [base64]: https://tools.ietf.org/html/rfc4648