README: update the list of tutorials

Author: faiface

README.md

@@ -22,6 +22,7 @@ covering several topics of Pixel. Here's the content of the tutorial parts so fa
 - [Drawing efficiently with Batch](https://github.com/faiface/pixel/wiki/Drawing-efficiently-with-Batch)
 - [Drawing shapes with IMDraw](https://github.com/faiface/pixel/wiki/Drawing-shapes-with-IMDraw)
 - [Typing text on the screen](https://github.com/faiface/pixel/wiki/Typing-text-on-the-screen)
+- [Using a custom fragment shader](https://github.com/faiface/pixel/wiki/Using-a-custom-fragment-shader)
 
 ## [Examples](https://github.com/faiface/pixel-examples)