Update and rewrite CONTRIBUTING
This file hadn't kept up with reality, and needs updating for Gitlab. Take the opportunity to rewrite it.
Much of the text, particularly about commit messages, was taken from Wayland's contributing guide (thanks to Ander Conselvan de Oliveira, Bryce Harrington, Eric Engestrom, Pekka Paalanen and Daniel Stone).
Please see https://gitlab.freedesktop.org/smcv/dbus/blob/contributing/CONTRIBUTING.md for a HTML rendering of this file, which is similar to what the main dbus repository will offer as a "Contributing guide" after this merge request lands. Reviewing the diff is not necessarily very useful: this is basically a rewrite.