ieee802154: add documentation about our stack

Add MAINTAINERS entry and a small text describing our stack interfaces,
how to hook the drivers, etc.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: David S. Miller <davem@davemloft.net>