doc: update contribute.html to clarify replying to reviews via email

Update golang/go#22120

Change-Id: Ie7dbd0e7b01b116c960243a8cd3fa9fd121f317d
Reviewed-on: https://go-review.googlesource.com/68021Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

doc/contribute.html

@@ -681,8 +681,8 @@ reviewers asking them to visit the issue's URL and make comments on the change.
 When done, the reviewer adds comments through the Gerrit user interface
 and clicks "Reply" to send comments back.
 You will receive a mail notification when this happens.
-You must reply through the web interface.
-(Unlike with the old Rietveld review system, replying by mail has no effect.)
+You may reply through the web interface or
+<a href="https://gerrit-review.googlesource.com/Documentation/intro-user.html#reply-by-email">via email</a>.
 </p>
 
 <h3 id="revise">Revise and resend</h3>
@@ -696,8 +696,8 @@ all the changes and comments made in the single URL.
 </p>
 
 <p>
-You must respond to review comments through the web interface.
-(Unlike with the old Rietveld review system, responding by mail has no effect.)
+You may respond to review comments through the web interface or
+<a href="https://gerrit-review.googlesource.com/Documentation/intro-user.html#reply-by-email">via email</a>.
 </p>
 
 <p>