contributing: about re-sending patches

This is what is generally expected from people who re-send patches,
whether the patches are their own or not.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>