manual: contribute.txt: add patch revision section

Improve the contribute manual section by adding an explanation about patch
review and version.

The section now provides advices in how to respond maintainers requests and how
to proceed on replying them.

[Thomas: further small modifications.]
Signed-off-by: Vinicius Tinti <viniciustinti@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>