Add README

linux_driver/README.md

+This kernel module is meant to work with the Arduino sketch in the directory above.
+The Arduino sketch operates with a set of i2c registers and generates GPIO pin interrupts.
+
+This module has only been tested with a Sierra Wireless WP7702 module, but should work fine
+anywhere.  It uses Platform Data instead of Device Tree Overlays.  This is done because the
+i2c bus used in the Sierra Wireless mangOH that this was tested with is off of an I2C switch
+and they chose not to the Device Tree.