Added Raspberry Pi User Guide

This User Guide is written by Eben Upton, Co-Creator of the Raspberry Pi

free-programming-books.md

@@ -138,6 +138,7 @@
 * [QML](#qml)
 * [R](#r)
 * [Racket](#racket)
+* [Raspberry Pi] (#rpi)
 * [REBOL](#rebol)
 * [Ruby](#ruby)
     * [RSpec](#rspec)
@@ -1541,6 +1542,8 @@ See also [TeX](#tex)
 * [Programming Languages: Application and Interpretation](http://cs.brown.edu/courses/cs173/2012/book/index.html)
 * [The Racket Guide](http://docs.racket-lang.org/guide/index.html)
 
+### Raspberry Pi
+* [Raspberry Pi Users Guide](http://www.cs.unca.edu/~bruce/Fall14/360/RPiUsersGuide.pdf) (PDF) - Eben Upton
 
 ### REBOL
 * [Learn REBOL](http://www.lulu.com/shop/nick-antonaccio/learn-rebol/ebook/product-17383182.html) - Nick Antonaccio