Add CONTRIBUTING.md document

Taken from Pekka's wayland/wayland@630c25f4c160 and follow-ups, use
Wayland's CONTRIBUTING document as a basis for Weston.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>