contributing: use Gitlab merge request workflow

The experience from Weston shows that the Gitlab merge request based workflow
works really well. Recently there have also been issues with the mailing list
that have made the email based workflow more painful than it used to be. Those
issues might have been temporary or occasional, but they probably are only
going to increase.

The MR workflow is different, it has its issues
(freedesktop/freedesktop#74) and we
likely lose the explicit Reviewed-by etc. tags from commit messages, but it is
also much easier to work with: no more whitespace damaged patches, lost email,
setting up git-send-email; we gain automated CI before any human reviewer even
looks at anything, and people can jump in to an ongoing discussion even if they
weren't subscribed before.

If you still want email, you can subscribe to that selectively(!) in Gitlab
yourself.

This text has been copied from Weston's CONTRIBUTING.md of the 5.0.91 release
and slightly altered for Wayland.

Fixes: #49

v2: fixed two left-over mentions of Weston

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
v1 Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Scott Anderson <scott.anderson@collabora.com>
Acked-by: Ian Ray <ian.ray@ge.com>
Acked-by: Derek Foreman <derek.foreman.wayland@gmail.com>
1 job for master in 2 minutes and 43 seconds (queued for 24 minutes and 39 seconds)
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