Add Scratchapixel.com to resources (#63)

Author: ShadowMitia

resources/index.html

@@ -82,6 +82,7 @@
 		* [Finding Your Home in Game Graphics Programming](http://alextardif.com/LearningGraphics.html)
 		* [Game Math](https://gamemath.com/)
 		* [A Comparison of Modern Graphics APIs](https://alain.xyz/blog/comparison-of-modern-graphics-apis)
+		* [Scratchapixel](https://www.scratchapixel.com)
 
 	</code>