doc/user: expand the Contributing document

This is mostly taken from the Weston Contributing.md document.
Main changes from there are:
- more detailed step-by-step on how to create a MR
- commit history/messages in two sections
- s/Weston/libinput/

I skipped the Review/Commit Rights sections for now until there's some demand
for it. Same with the Licensing/Stabilising for releases sections.

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