Expanded Documentation for bare css option

As per the remaining notes on #71, I have expanded the documentation for the tablesaw.bare.css option to explain it in greater detail.

This has been done on both the readme and the bare css demo page itself. Only a sentence or two expansion in each place, I didn't want to write a treatise here, but hopefully this helps alleviate confusion such as that which prompted @PattyToland to open this issue in the first place.

One quick note, I don't see the explainer text on the current bare demo page as seen in the repo on the live one (http://filamentgroup.github.io/tablesaw/demo/bare.html). Is that something in the markup that needs resolving, or just something infrastructural pushing it live?

Thanks!