docs/manual: update contribute.txt to cover get-developers

This commit updates the contribute.txt part of the manual to tell
people to use get-developers to get the appropriate "git send-email"
command when sending patches.

[Peter: use --cc instead of --to as suggested by Yann/Arnout]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>