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).

Signed-off-by: Simon McVittie <smcv@collabora.com>