doc: add important info for would-be go contributors.

Link to the "installing from source" docs instead of the "installing" docs.

Remind would-be hacker to switch to the default branch from the release branch.

R=rsc
CC=golang-dev
https://golang.org/cl/6559043

doc/contribute.html

@@ -7,7 +7,7 @@
 <p>
 This document explains how to contribute changes to the Go project.
 It assumes you have installed Go using the
-<a href="/doc/install">installation instructions</a> and
+<a href="/doc/install/source">installation instructions</a> and
 have <a href="code.html">written and tested your code</a>.
 (Note that the <code>gccgo</code> frontend lives elsewhere;
 see <a href="gccgo_contribute.html">Contributing to gccgo</a>.)

doc/install-source.html

@@ -150,6 +150,18 @@ Then check out the repository:</p>
 $ hg clone -u release https://code.google.com/p/go
 </pre>
 
+<h2 id="head">(Optional) Switch to the default branch</h2>
+
+<p>If you intend to modify the go source code, and
+<a href="/doc/contribute.html">contribute your changes</a>
+to the project, then move your repository
+off the release branch, and onto the default (development) branch.
+Otherwise, skip this step.</p>
+
+<pre>
+$ hg update default
+</pre>
+
 <h2 id="install">Install Go</h2>
 
 <p>