docs/manual/contribute.txt: add section for preparing patch series

In subsequent patches, we will add more explanation about how to
prepare patches, so it will be worthwhile to have a separate section
for the series preparation.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas: rewrap to our normal formatting practice.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>