More detailed documentation

[DmitryAstafyev]

Thanks for the nice library, it's easy and clean in many aspects.

I guess it's related to #10, but in any way would be great to have more detailed documentation with descriptions of such things like derive, implementation of try_to_js and convert_to_rust, and so on.

[sehz]

Thanks for feedback. I will try to add more docs to API