Choose a License for book contents

[ickk]

Using CC0 for example code is pretty standard, but we need to agree on a license for the rest of the content in this repo.

[leprechaunt33]

I'm happy to go with CC0 for the code and CC-BY for the tutorial content itself. If I supply a full source folder for the example I'd need to tweak the GLB and other assets to make them release ready (removing personal content like key signing info and changing out the image in the photo frame).

So far the content in the fork is a good start but probably needs to be finessed a bit before its final. Additional material on colliders and a conclusion as well as more on the input context and render system would probably be a good idea.