[RFKILL]: Add rfkill documentation

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: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>