doc: mention 'hg update default' in contribution guidelines

Fixes #3634.

R=golang-dev, bsiegert, rsc
CC=golang-dev
https://golang.org/cl/6270047

doc/contribute.html

@@ -163,6 +163,19 @@ can use that nickname as a shorthand for naming reviewers and the CC list.
 For example, <code>rsc</code> is an alias for <code>rsc@golang.org</code>.
 </p>
 
+<h3>Switch to the default branch</h3>
+
+<p>
+Most Go installations use a release branch, but new changes should
+only be made to the default branch. (They may be applied later to a release
+branch as part of the release process.)
+Before making a change, make sure you use the default branch:
+</p>
+
+<pre>
+$ hg update default
+</pre>
+
 <h3>Make a change</h3>
 
 <p>