contributing: add review guidelines

This sets up the standards for patch review, and defines when a patch
can be merged. I believe these are the practises we have been using
already for a long time, now they are just written down explicitly.

It's not an exhaustive list of criteria and likely cannot ever be, but
it should give a good idea of what level of review we want to have.

It has been written in general terms, so that we can easily apply the
same text not just to Wayland, but also Weston and other projects as
necessary.

This addition is not redundant with
https://wayland.freedesktop.org/reviewing.html

 .

The web page is a friendly introduction and encouragement for people to
get involved. The guidelines here are more specific and aimed for people
who seek commit rights or maintainership.

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>