Clarify the procedure developer must follow to request privileges to
run tests on Freedesktop gitlab CI.

This measure was added to avoid untrusted people to misuse the
infrastructure.
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Link: https://lore.kernel.org/r/20231024004525.169002-11-helen.koike@collabora.comSigned-off-by: Maxime Ripard <mripard@kernel.org>