Add docs.gl to resources (#53)

* Add docs.gl to resources

docs.gl is a super nifty website I wouldn't want to go without when fiddling with OpenGL.

* Order alphabetically

Author: bubba2k

resources/index.html

@@ -41,6 +41,7 @@
 		* [Metal Specification](https://developer.apple.com/documentation/metal)
 
 		## OpenGL
+		* [docs.gl - OpenGL API Documentation](http://docs.gl/)
 		* [Guide to Modern OpenGL Functions](https://github.com/fendevel/Guide-to-Modern-OpenGL-Functions)
 		* [Learn OpenGL](https://learnopengl.com/)
 		* [OpenGL 4.6 Specification](https://www.khronos.org/registry/OpenGL/specs/gl/glspec46.core.pdf)