docs: Reference Contributor Covenant

All fd.o projects are now covered by the Contributor Covenant. Include a
reference to this in the Contributing doc, making it clear that we are
all expected to behave like human beings.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Jonas Ådahl <jadahl@gmail.com>