Improve documentation to include visual of what example app should look like?

[JulianKnodt]

I was hoping to learn how to write rust-gpu shaders, so I was going through the tutorial, and have to run the CPU version on my old mac air.

I think for rendering, it may be useful to include images, as I'm not sure if this output looks correct:

Specifically it may be useful to include an image after the initial build steps to make sure that it's running correctly.