Add the documentation for the new sysfs interface of dell-laptop
that allows to configure the keyboard illumination on Dell systems.
Signed-off-by: NGabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: NPali Rohár <pali.rohar@gmail.com>
Signed-off-by: NDarren Hart <dvhart@linux.intel.com>