Build environment requires proper checkout of project

This clarifies the importance of properly setting a Go build environment when
building targets. Typically, users seem to editorialize the checkout location,
either ignoring the first section or have limited experience with the Go
development environment. We clarify the checkout requirements and point to the
documentation on how to setup Go.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/building.md

@@ -1,10 +1,11 @@
-page_title: Build the development environment
-page_description: Explains how to build the distribution project
-page_keywords: registry, service, images, repository
-
 # Build the development environment
 
-If a go development environment is setup, one can use `go get` to install the
+The first prequisite of properly building distribution targets is to have a Go
+development environment setup. Please follow [How to Write Go Code](https://golang.org/doc/code.html)
+for proper setup. If done correctly, you should have a GOROOT and GOPATH set in the
+environment.
+
+If a Go development environment is setup, one can use `go get` to install the
 `registry` command from the current latest:
 
 ```sh
@@ -19,6 +20,11 @@ $ $GOPATH/bin/registry -version
 $GOPATH/bin/registry github.com/docker/distribution v2.0.0-alpha.1+unknown
 ```
 
+> __NOTE:__ While you do not need to use `go get` to checkout the distribution
+> project, for these build instructions to work, the project must be checked
+> out in the correct location in the `GOPATH`. This should almost always be
+> `$GOPATH/src/github.com/docker/distribution`.
+
 The registry can be run with the default config using the following
 incantantation: