Update README with the master copy location

Author: bjpirt

README

@@ -1,3 +1,5 @@
 A library for Arduino to make it easier to talk to Cosm (the service formerly known as Pachube).
 
+The master copy of this code lives at http://github.com/cosm/cosm-arduino so make sure you're not looking at a branched copy elsewhere.
+
 Requires the HTTP Client library in https://github.com/arduino/Arduino/pull/69 - which should be included in Arduino 1.0.1.