doc: Added link to 'go help gopath'

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/2265Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/code.html

@@ -127,6 +127,11 @@ to your <code>PATH</code>:
 $ <b>export PATH=$PATH:$GOPATH/bin</b>
 </pre>
 
+<p>
+To learn more about setting up the <code>GOPATH</code> environment variable,
+please see
+<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>go help gopath</code></a>
+</p>
 
 <h3 id="PackagePaths">Package paths</h3>