add "The Python GTK+ 3 Tutorial"

free-programming-books.md

@@ -1147,6 +1147,7 @@ See also [TeX](#tex)
 * [The Linux Development Platform (PDF)](http://www.informit.com/content/downloads/perens/0130091154.pdf) (PDF)
 * [The Linux Kernel Module Programming Guide](http://tldp.org/LDP/lkmpg/2.6/html/)
 * [The Linux System Administrator's Guide](http://www.tldp.org/LDP/sag/html/index.html)
+* [The Python GTK+ 3 Tutorial](http://python-gtk-3-tutorial.readthedocs.org/en/latest/)
 * [Ubuntu Pocket Guide and Reference](http://www.ubuntupocketguide.com/index_main.html)
 * [Ubuntu Server Guide](https://help.ubuntu.com/13.10/serverguide/serverguide.pdf)
 * [Upstart Intro, Cookbook and Best Practises](http://upstart.ubuntu.com/cookbook/)