Added interactive OpenGL Shading Language Tutorial (#2658)

* Added interactive OpenGL Shading Language Tutorial

* fix spacing

* trailing slash

free-programming-interactive-tutorials-en.md

@@ -9,6 +9,7 @@
 * [Erlang](#erlang)
 * [F#](#f-sharp)
 * [Git](#git)
+* [GLSL](#glsl)
 * [Go](#go)
 * [Haskell](#haskell)
 * [HTML / CSS](#html--css)
@@ -83,6 +84,11 @@
 * [Try Git](http://try.github.io)
 
 
+### GLSL
+
+* [The Book of Shaders](https://thebookofshaders.com)
+
+
 ### Go
 
 * [Go Koans](https://github.com/cdarwin/go-koans)