Improve documentation about mailing lists.

This CL fixes the first part of the issue. Staged at:

https://support-dot-dart-lang.appspot.com/support/

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1133
R=kevmoo@google.com

Review URL: https://chromiumcodereview.appspot.com/715693002