Added information about the need to include the Arduino SPI library when using the Due

Author: Lauszus

README.md

@@ -66,6 +66,7 @@ By default serial debugging is disabled. To turn it on simply change ```ENABLE_U
 Currently the following boards are supported by the library:
 
 * All official Arduino AVR boards (Uno, Duemilanove, Mega, Mega 2560, Mega ADK, Leonardo etc.)
+    * If you are using the Arduino Due, then you must include the Arduino SPI library as well like so: ```#include <SPI.h>``` to your .ino file.
 * Arduino Due
 * Teensy (Teensy++ 1.0, Teensy 2.0, Teensy++ 2.0, and Teensy 3.x)
 	* Note if you are using the Teensy 3.x you should download this SPI library as well: <https://github.com/xxxajk/spi4teensy3>. You should then add ```#include <spi4teensy3.h>``` to your .ino file.