HID: lg4ff - Document sysfs interface for range setting

Adds documentation of the sysfs interface used to set the wheel range.
Signed-off-by: Michal Malý <madcatxster@gmail.com>
Signed-off-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/ABI/testing/sysfs-driver-hid-logitech-lg4ff

+What:		/sys/module/hid_logitech/drivers/hid:logitech/<dev>/range.
+Date:		July 2011
+KernelVersion:	3.2
+Contact:	Michal Malý <madcatxster@gmail.com>
+Description:	Display minimum, maximum and current range of the steering
+		wheel. Writing a value within min and max boundaries sets the
+		range of the wheel.