doc: mention moderation delay in contributing docs

This sometime worries new contributors.
Hopefully mentioning it here will help.

Fixes #11300.

Change-Id: Ica7f10d749731704ac6a2c39c7dcba389996011e
Reviewed-on: https://go-review.googlesource.com/11236Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -386,7 +386,8 @@ Unless explicitly told otherwise, such as in the discussion leading
 up to sending in the change list, it's better not to specify a reviewer.
 All changes are automatically CC'ed to the
 <a href="https://groups.google.com/group/golang-codereviews">golang-codereviews@googlegroups.com</a>
-mailing list.
+mailing list. If this is your first ever change, there may be a moderation
+delay before it appears on the mailing list, to prevent spam.
 </p>
 
 <p>