Adding MagPi Learn to Code With C (#2109)

Via official tweet https://twitter.com/Raspberry_Pi/status/786526873396342784

free-programming-books.md

@@ -819,6 +819,7 @@
 * [C Programming](https://en.wikibooks.org/wiki/Programming%3AC) - Wikibooks
 * [Deep C](http://www.slideshare.net/olvemaudal/deep-c)
 * [Essential C](http://cslibrary.stanford.edu/101/EssentialC.pdf) (PDF)
+* [Learn to Code With C - The MagPi Essentials](https://www.raspberrypi.org/magpi-issues/Essentials_C_v1.pdf) (PDF)
 * [Modern C](http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf) (PDF)
 * [Object-Oriented Programming With ANSI-C](http://www.planetpdf.com/codecuts/pdfs/ooc.pdf) (PDF)
 * [Programming in C](http://ee.hawaii.edu/~tep/EE160/Book/PDF/) - Bharat Kinariwala & Tep Dobry