V4L/DVB: V4L2: Add a document describing the videobuf layer

Videobuf is a moderately complex API which most V4L2 drivers should use,
but its documentation is...sparse.  This document attempts to improve the
situation.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>