Add CSS: Selectors docs

@julienbenatar @karlsluis @abigailricarte @dodeca @mbildner @spucci @samirrayani 

Please review and comment.

If after reading this document you feel that an Ace series on the subject would be helpful, please leave a comment to that effect.
Dumb question, why doubles (__ and --) instead of singles (_ and -)? Further differentiation from other libraries?

I love how tight, scannable, and direct this doc is, Mike. Really awesome!