doc/contribute: add instructions to fix codereview on windows

Tell Windows users how to update Mercurial's library.zip to add some missing dependencies.

Fixes #4745.

R=golang-dev, patrick.allen.higgins, minux.ma, adg
CC=golang-dev
https://golang.org/cl/7558043

doc/contribute.html

@@ -117,6 +117,13 @@ enabled for your checkout in <code>$GOROOT</code>, the remainder of this
 document assumes you are inside <code>$GOROOT</code> when issuing commands.
 </p>
 
+<p>
+Windows users may need to perform extra steps to get the code review 
+extension working. See the
+<a href="https://code.google.com/p/go-wiki/wiki/CodeReview">CodeReview page</a> 
+on the <a href="http://code.google.com/p/go-wiki/wiki">Go Wiki</a> for details.
+</p>
+
 <h3>Log in to the code review site.</h3>
 
 <p>