CONTRIBUTING.md: Inform users that they'd need to ask for perms

Due to the latest developments fdo no longer allows new users to fork
the Weston project so let's inform users about that.

This also swaps 'Finding something to work on' with 'Sending patches'
paragraph as the first thing users need to look into.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>