Add udev rules files based on README

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

90-backlight.rules

@@ -0,0 +1,2 @@
+ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
+ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"