Added a minimal README to the library

lib/README.md

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+# PD Buddy Firmware Library
+
+This library implements a subset of the USB Power Delivery Specification,
+Revision 2.0, Version 1.3, and Revision 3.0, Version 1.1, for microcontrollers
+running [ChibiOS][] and connected to an [FUSB302B][] USB Power Delivery PHY.
+
+[ChibiOS]: http://www.chibios.org/
+[FUSB302B]: http://www.onsemi.com/PowerSolutions/product.do?id=FUSB302B