dtc and libfdt have been using Signed-off-by lines (as used in the Linux
kernel) for some time, like a lot of open source projects.  However
Uwe Kleine-König pointed out we never really stated what they mean in our
context.

Add information on what the S-o-b line means in CONTRIBUTING.md - this is
essentially a quote of the same information from the kernel documentation,
with some tweaks to make sense in the new context.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>