Add GLib/GTK+ Development Platform book. (#2681)

* Add GLib/GTK+ Development Platform book.

The books is licensed under a Creative Commons Attribution-ShareAlike 4.0
International License:
https://creativecommons.org/licenses/by-sa/4.0/

* Put link in order.

* Add pdf indicator.

free-programming-books.md

@@ -255,6 +255,7 @@
 * [Best of Smashing Magazine](http://anniversary.smashingmagazine.com/best-of-smashing-magazine.zip)
 * [Programming with gtkmm 3](https://developer.gnome.org/gtkmm-tutorial/stable/)
 * [Search User Interfaces](http://searchuserinterfaces.com/book/) - Marti A. Hearst
+* [The GLib/GTK+ Development Platform](https://people.gnome.org/~swilmet/glib-gtk-dev-platform.pdf) - Sébastien Wilmet (PDF)
 
 
 ### Language Agnostic