doc: move Contributing

Gitlab expects a CONTRIBUTING.md in the root directory, so move our
guide there.

Conversion to proper markup is a follow-up patch.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>