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.
Isn't it some sort of "noise" in this page to write down the bad practice? The good practice seems clear enough in my opinion.