model.md

[pixelzoom]

Complete the model description in model.md, then have @arouinfar review it.

Unfortunately, there's nothing from the Java version to be reused here.

[pixelzoom]

@arouinfar this is ready for review. Let me know if you'd like to change anything. Feel free to correct any typos. There's no hurry on reviewing this. It's good enough for the code reviewer, and can easily be changed at any point during QA.

[arouinfar]

@pixelzoom this looks really good. I fixed a few typos, but I don't have any other comments.