doc/contribute: mention 'hg help codereview'

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6932053

doc/contribute.html

@@ -110,6 +110,11 @@ The <code>username</code> information will not be used unless
 you are a committer (see below), but Mercurial complains if it is missing.
 </p>
 
+<p>
+After adding the extension, <code>hg help codereview</code>
+will show documentation for its commands.
+</p>
+
 <h3>Log in to the code review site.</h3>
 
 <p>