docs: add links to clarify patch mailing section

docs: add links to clarify patch mailing section

* Changed "Mesa mailing list" to "mesa-dev mailing list" to clarify
  which list patches should be sent to

* Added an explicit link to
  https://lists.freedesktop.org/mailman/listinfo/mesa-dev to show
  where to subscribe to the list

* Added a link to https://git-scm.com/docs/git-send-email to help new
  users of that command

v2: add signed-off-by

Signed-off-by: Nicholas Bishop <nicholasbishop@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>