doc: mention multi-change branches in contribute.html

Fixes #13651.

Change-Id: I1d21b49e2b5bc6c507eb084d6d2553e5a9c607cf
Reviewed-on: https://go-review.googlesource.com/19552Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/contribute.html

@@ -198,9 +198,13 @@ prints help text, not an error.
 </p>
 
 <p>
-Note to Git aficionados: The <code>git-codereview</code> command is not required to
+<b>Note to Git aficionados:</b>
+The <code>git-codereview</code> command is not required to
 upload and manage Gerrit code reviews. For those who prefer plain Git, the text
-below gives the Git equivalent of each git-codereview command. If you do use plain
+below gives the Git equivalent of each git-codereview command.
+</p>
+
+<p>If you do use plain
 Git, note that you still need the commit hooks that the git-codereview command
 configures; those hooks add a Gerrit <code>Change-Id</code> line to the commit
 message and check that all Go source files have been formatted with gofmt. Even
@@ -208,6 +212,12 @@ if you intend to use plain Git for daily work, install the hooks in a new Git
 checkout by running <code>git-codereview</code> <code>hooks</code>.
 </p>
 
+<p>
+The workflow described below assumes a single change per branch.
+It is also possible to prepare a sequence of (usually related) changes in a single branch.
+See the <a href="https://golang.org/x/review/git-codereview">git-codereview documentation</a> for details.
+</p>
+
 <h3 id="git-config">Set up git aliases</h3>
 
 <p>