README: reduce it to a single page and refer to other documentation

As discussed on the BR developer meeting at FOSDEM, the README should be
very short and instead refer to other documentation: the manual, the
website, the mailing list, the IRC channel.

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