doc/contribute.html: mention hg mail during hg change discussion

People keep not reading all the way to the bottom of the doc
and not running hg mail.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7137057

doc/contribute.html

@@ -289,6 +289,11 @@ to add <code>somefile</code> to CL 99999, and use <code>hg file -d 99999 somefil
 <code>somefile</code> from the CL.
 </p>
 
+<p>
+Creating the change uploads a copy of the diff to the code review server, but it does not
+notify anyone about it. To do that, you need to run <code>hg mail</code> (see below).
+</p>
+
 <p>
 You can see a list of your pending changes by running <code>hg pending</code> (<code>hg p</code> for short).
 </p>