docs/manual/contribute.txt: add formatting patches section

Thomas P. has sent a few big feedback mails recently that explain how a
patch should be formatted. Indeed, this was not explained much in the
manual, so add a section that explains how patches should be formatted.
This is based heavily on the feedback that Thomas P. gave. Also,
specific examples for new packages and version bumps are added.

This will allow us to refer to
https://buildroot.org/manual.html#submitting-patches


in the future instead of composing long mails.

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>