doc: updated language about the AUTHORS/CONTRIBUTORS update

Reflect the process changes where AUTHORS and CONTRIBUTORS
files are updated automatically based on commit logs
and Google committers no longer need to do it manually
on the first contributors.

The documentation update will help to avoid requests to be
added from new contributors.

Change-Id: I67daae5bd21246cf79fe3724838889b929bc5e66
Reviewed-on: https://go-review.googlesource.com/10824Reviewed-by: Rob Pike <r@golang.org>

doc/contribute.html

@@ -624,10 +624,11 @@ and perhaps the <a href="/AUTHORS"><code>AUTHORS</code></a> file.
 defines who the Go contributors&mdash;the people&mdash;are;
 the <a href="/AUTHORS"><code>AUTHORS</code></a> file defines
 who &ldquo;The Go Authors&rdquo;&mdash;the copyright holders&mdash;are.
-The Go developers at Google will update these files when submitting
-your first change.
-In order for them to do that, you need to have completed one of the
-contributor license agreements:
+These files will be periodically updated based on the commit logs.
+
+<p>Gerrit serves as the gatekeeper and uses your e-mail address as the key.
+To send your first change to the Go project from a given address,
+you must have completed one of the contributor license agreements:
 <ul>
 <li>
 If you are the copyright holder, you will need to agree to the