README: more usage

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

README.md

@@ -10,14 +10,43 @@ vbatts@valse ~ (master) $ go get -u github.com/vbatts/git-validation
 
 ## usage
 
+The flags
 ```bash
-vbatts@valse ~/src/vb/git-validation (master) $ git-validation 
+vbatts@valse ~/src/vb/git-validation (master *) $ git-validation -h
+Usage of git-validation:
+  -D    debug output
+  -d string
+        git directory to validate from (default ".")
+  -list-rules
+        list the rules registered
+  -range string
+        use this commit range instead
+  -run string
+        comma delimited list of rules to run. Defaults to all.
+  -v    verbose
+```
+
+The default rule set are all run by default:
+```bash
+vbatts@valse ~/src/vb/git-validation (master) $ git-validation -list-rules
+"DCO" -- makes sure the commits are signed
+"short-subject" -- commit subjects are strictly less than 90 (github ellipsis length)
+```
+
+Comma delimited rules to run:
+```bash
+vbatts@valse ~/src/vb/git-validation (master) $ git-validation -run DCO,short-subject
+ * b243ca4 "README: adding install and usage" ... PASS
  * d614ccf "*: run tests in a runner" ... PASS
  * b9413c6 "shortsubject: add a subject length check" ... PASS
  * 5e74abd "*: comments and golint" ... PASS
  * 07a982f "git: add verbose output of the commands run" ... PASS
  * 03bda4b "main: add filtering of rules to run" ... PASS
  * c10ba9c "Initial commit" ... PASS
+```
+
+Verbosity shows each rule's output:
+```bash
 vbatts@valse ~/src/vb/git-validation (master) $ git-validation -v
  * d614ccf "*: run tests in a runner" ... PASS
   - PASS - has a valid DCO