Update README.md

Added detail for compiling with new version Arduino from version 3.1 /2.75

README.md

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 You can find lots of development information at the [ArduPilot development site](http://dev.ardupilot.com)
 
+#### To compile APM2.x Ardupilot after version 3.1 please follow the instructions found at 
+
+[Arduino Tools] (http://firmware.diydrones.com/Tools/Arduino/) 
+
+
 ## Getting the source
 
 You can either download the source using the "ZIP" button at the top