The existing go code document did not link to the GOPATH documentation.
This will link to it, in hopes of making it more discoverable.

Change-Id: Ie4ded2fdce08f412e4acbcc93acdd76f5791b84a
Reviewed-on: https://go-review.googlesource.com/2265


Reviewed-by: Andrew Gerrand <adg@golang.org>