Mention golang-dev as a default code review destination.

R=rsc
CC=r
https://golang.org/cl/160056

doc/contribute.html

@@ -334,6 +334,10 @@ The <code>Reviewer</code> line lists the reviewers assigned
 to this change, and the <code>CC</code> line lists people to
 notify about the change.
 These can be code review nicknames or arbitrary email addresses.
+If you don't know who is best to review the change, set the
+reviewer field to the
+<a href="http://groups.google.com/group/golang-dev">golang-dev@googlegroups.com</a>
+mailing list.
 </p>
 
 <p>