contributing: review rules for bugs

Half of the ideas came from Daniel but most of them are reworded, the
rest are my thoughts.

Mention compiler warnings specifically, and be more explicit on what
kind of code or bugs or bug fixes are acceptable or not. Clarify commit
scope.

v2: move the "In a patch series" rule to the bottom, reworded.

Cc: Daniel Stone <daniels@collabora.com>
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>