Consider adding more documentation

[pdesaulniers]

Related: mruby-zest/mruby-zest-build#10

This is a pretty vague issue, but I think mruby-zest could do with more documentation overall. A lot of things can be inferred by looking at its code directly, but it's good to have some easily readable document too. I guess we should first determine the scope that the documentation should have.

Having something akin to an auto-generated doxygen site would be better than nothing. It would improve the discoverability of Zest's API.

I think pretty much anything there is to know about the QML files should be written. How they work, which callbacks are predefined (onSetup, onMouseMove), etc.

Code-wise, it would be nice to add summaries at the top of classes and functions in a few places.

Maybe the library is still too young for this. I think it's something to keep in mind, though.