Add An Introduction to C & GUI Programming from Raspberry Pi Press/Simon Long (#3231)

* Adding Harvard GSE Scratch Book

* adding raspberry pi press book

* spelling sorting doesn't ignore "an" as first word of title

free-programming-books.md

@@ -922,6 +922,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 * [256-Color VGA Programming in C](http://www.brackeen.com/vga/) - David Brackeen
 * [A Tutorial on Pointers and Arrays in C](http://home.netcom.com/~tjensen/ptr/pointers.htm) - Ted Jensen [(PDF, Zipped HTML)](http://pweb.netcom.com/~tjensen/ptr/cpoint.htm)
+* [An Introduction to C & GUI Programming](https://www.raspberrypi.org/magpi-issues/C_GUI_Programming.pdf) - Simon Long (PDF)
 * [Bare-metal programming for ARM](https://github.com/umanovskis/baremetal-arm) - Daniels Umanovskis [(PDF)](http://umanovskis.se/files/arm-baremetal-ebook.pdf)
 * [Beej's Guide to C Programming](http://beej.us/guide/bgc/) - B. Hall
 * [Beej's Guide to Network Programming - Using Internet Sockets](http://beej.us/guide/bgnet/) - B. Hall