Merge pull request #66 from jnuserful/master

Added Programming with gtkmm and GUI category

free-programming-books.md

 ###Index
 * [Meta-Lists](#meta-lists)
+* [Graphics user interfaces](#graphics-user-interfaces)
 * [Graphics Programming](#graphics-programming)
 * [Language Agnostic](#language-agnostic)
 * [Ada](#ada)
@@ -106,6 +107,10 @@
 * [ShaderX series](http://tog.acm.org/resources/shaderx/)
 * [Tutorials for modern OpenGL](http://www.opengl-tutorial.org/)
 
+###Graphics User Interfaces
+
+* [Programming with gtkmm 3](https://developer.gnome.org/gtkmm-tutorial/stable/)
+ 
 
 ###Language Agnostic