This file hadn't kept up with reality, and needs updating for Gitlab.
Take the opportunity to rewrite it.

Much of the text here 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>