Improve README, add a GitBook?

[maciejhirsz]

I think the current the examples in README are trying to explain too many things at once. The first example in particular is pretty odd. I need to rework it with some very very simple examples, maybe just one, showing how parsing, checking a value, mutating and stringifying works.

On top of that, a GitBook talking about how to use the library might be a nice addition.