docs: add note about sending merge-requests from forks

docs: add note about sending merge-requests from forks

Sending MRs from the main Mesa repository increase clutter in the
repository, and decrease visibility of project-wide branches. So it's
better if MRs are sent from forks instead.

Let's add a note about this, in case its not obvious to everyone.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>