Add comprehensive developer docs #171

I think the codebase is not really suitable for reading and editing as it is now, as a minimum I would want a table and/or docstrings for mapping CSS features to GenericTranslator methods and back and a doc about the logic flow between the CSS selector input and the XPath result. It would also be nice to have (if possible and not too unwieldy) a table(s) listing all features of CSS3/4 and marking which are supported, not implemented or can't be implemented, and some kind of table of a manually written examples covering all features we support (like the tests but more comprehensive, and hopefully synced with them).