Add note on cellular noise

proceduralTexture/SUMMARY.md

@@ -1,3 +1,3 @@
 *[kynd](http://www.kynd.info) Nov 20, 2016*
 
-Shaders are often used to create realistic surfaces of natural or artificial material such as wood, marble, granite, metal, stone, etc. without using photographs or pre-rendered images. Here are demonstrations of some basic techniques. For the sake of simplicity, all the examples are based on the same random and noise functions so that you can focus on the main part of the algorithms. Once you get the ideas, try replacing these functions with with others from [Random](http://thebookofshaders.com/10/) and [Noise](http://thebookofshaders.com/11/) chapters, making them more realistic, and optimizing the performance.
+Shaders are often used to create realistic surfaces of natural or artificial material such as wood, marble, granite, metal, stone, etc. without using photographs or pre-rendered images. Here are demonstrations of some basic techniques. For the sake of simplicity, all the examples are based on the same random and noise functions so that you can focus on the main part of the algorithms. Once you get the ideas, try replacing these functions with with others from [Random](http://thebookofshaders.com/10/) and [Noise](http://thebookofshaders.com/11/) chapters, making them more realistic, and optimizing the performance. [Cellular Noise](http://thebookofshaders.com/12/) is very useful for making verious textures like animal skin, plant, rock surface, etc. Try it out.