contributing: commit rights

This has been copied from
https://cgit.freedesktop.org/xorg/app/intel-gpu-tools/tree/CONTRIBUTING?id=eccae1360d6d01e73c6af2bd97122cef708207ef
and slightly edited to better with Wayland and Weston.

The intention is to make it easier to give out commit access to new
people, let them know what is expected of them, and help the community
to grow. Hopefully this will in time improve the patch review throughput
and timeliness.

The original text was introduced in
https://cgit.freedesktop.org/xorg/app/intel-gpu-tools/commit/CONTRIBUTING?id=0350f0e7f6a0e07281445fc3082aa70419f4aac7



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Matheus Santana <embs@cin.ufpe.br>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>