Documentation: Add reference to the Firmata Linux Kernel module

Adds a reference to the (experimental) Linux Kernel Module firmata_mod,
which allows users to use standard Linux userspace APIs/tools to access GPIO,
I2C or SPI devices made available on an Arduino attached to a Linux host PC via
a serial interface and running the Firmata firmware. In addition, existing
Linux Kernel modules for devices using I2C/SPI can also be associated with
devices on the Arduino.

Author: logicog

readme.md

@@ -89,6 +89,8 @@ Most of the time you will be interacting with Arduino with a client library on t
   * [https://github.com/NullMember/PDFirmata](https://github.com/NullMember/PDFirmata)
 * Common Lisp
   * [https://github.com/cjfuller/cl-firmata](https://github.com/cjfuller/cl-firmata)
+* Linux Kernel Module
+  * [https://github.com/logicog/firmata_mod]
 
 Note: The above libraries may support various versions of the Firmata protocol and therefore may not support all features of the latest Firmata spec nor all Arduino and Arduino-compatible boards. Refer to the respective projects for details.