Add simple GPGPU "prelude" exercise.

Just had a quick go at this one, feedback on content would be appreciated :)

Could potentially lead onto the following topics, among others:

* Cellular Automata
* Feedback Effects: Separating Logic/Rendering
* Feedback Effects: Floating Point Textures
* Using `gl.POINTS` to create a particle system

Thanks!
@chrisdickinson nope, it's here :)