doc/install: mention GOPATH and point to code.html

Fixes #3469.

R=golang-dev, kevlar, rsc, r
CC=golang-dev
https://golang.org/cl/6304049

doc/install-source.html

@@ -218,6 +218,12 @@ hello, world
 If you see the "hello, world" message then Go is installed correctly.
 </p>
 
+<h2 id="gopath">Set up your work environment</h2>
+
+<p>
+The document <a href="/doc/code.html">How to Write Go Code</a> explains how to
+set up a work environment in which to build and test Go code.
+</p>
 
 <h2 id="community">Community resources</h2>
 

doc/install.html

@@ -227,15 +227,17 @@ hello, world
 If you see the "hello, world" message then your Go installation is working.
 </p>
 
-<h2 id="next">What's next</h2>
+<h2 id="gopath">Set up your work environment</h2>
 
 <p>
-Start by taking <a href="http://code.google.com/p/go-tour/">A Tour of Go</a>.
+The document <a href="/doc/code.html">How to Write Go Code</a> explains how to
+set up a work environment in which to build and test Go code.
 </p>
 
+<h2 id="next">What's next</h2>
+
 <p>
-For more detail about the process of building and testing Go programs
-read <a href="/doc/code.html">How to Write Go Code</a>.
+Start by taking <a href="http://code.google.com/p/go-tour/">A Tour of Go</a>.
 </p>
 
 <p>