Add a documentation file which contains
a short description about rfkill with some
notes about drivers and the userspace interface.

Changes since v1 and v2:
 - Spellchecking
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Acked-by: NDmitry Torokhov <dtor@mail.ru>
Acked-by: NRandy Dunlap <randy.dunlap@oracle.com>