README: add reference to submitting-patches

In the hope of improving the quality of patches sent by newcomers,
add a reference to the submitting-patches section of the manual
to the top-level README file.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>