Add documentation explaining how this template project works

[jstarry]

[xlpxxx]

look elm document

[KarlHeitmann]

I am starting with Rust, and I came here. When I've seen this issue I thought maybe I could take it, but when I've read @36k-wild-monkey comment, I've started studying Elm, and now I understand very well how does this template works. If you are comfortable with JSX, you know some rust basic, and a little bit of elm, this template is pretty self explanatory.