contributing: format as markdown

Format for nice viewing through Gitlab.

Conduct and Licensing were raised to first level headings as they are
not technical guidelines for contributing patches. It's nice to use the
first level headings more.

Reformat patchwork link and add Xorg patchwork link for Xwayland.

v2: Unfortunately Gitlab harcodes a tab character to mean 4 spaces, so we
cannot reasonably spell the coding style examples correctly. Hence, tab
characters have been replaced with eight spaces so that they at least
look right in both the file and through gitlab web UI.

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>