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.
Alternate implementations of BEM use dash-delimited names instead of camelCase, so double `--` or `__` were used to differentiate from single `-` between words. Here, though it's purely for readability—it's easier to see the difference between double `--` and `__` than single `-` and `_` when scanning.

And if we decide to switch from camelCase to dash-delimited casing in the future, we won't be totally screwed.