can: add broadcast manager documentation

This patch adds documentation about the broadcast manager. It's based on Brian
Thorne's initial patch http://marc.info/?l=linux-can&m=138119382015496&w=2 and
Daniele Venzano's work http://brownhat.org/docs/socketcan.html

 .

Signed-off-by: Brian Thorne <hardbyte@gmail.com>
Cc: Daniele Venzano <linux@brownhat.org>
Cc: Andre Naujoks <nautsch2@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>