docs: Document GitLab merge request process (email alternative)

docs: Document GitLab merge request process (email alternative)

This documents a process for using GitLab Merge Requests as an second
way to submit code changes for Mesa. Only one of the two methods is
allowed for each patch series.

We will *not* require all patches to be emailed. Some code changes may
be reviewed and merged without any discussion on the mesa-dev email
list.

v2:
 * No longer require email. Allow submitter to choose email or a
   GitLab merge request.
 * Various feedback from Brian, Daniel, Dylan, Eric, Erik, Jason,
   Matt, Michel and Rob.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Rob Clark <robdclark@gmail.com>