Contributing: explain Patchwork

Contributing: explain Patchwork

Add general guidelines for using Patchwork, as we heavily rely on it
nowadays.

v2:

- mention also Xwayland and libinput patch management
- reword "if not found in Patchwork"
- reword "Not applicable"
- mention pwclient

Cc: Bryce Harrington <bryce@osg.samsung.com>
Cc: Olivier Fourdan <ofourdan@redhat.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>