Update README.md with serial comma
Update README.md with serial comma

Update README.md to include a serial comma to better resolve ambiguity between the three terms HTML, SVG, and CSS. [Wikipedia](http://en.wikipedia.org/wiki/Serial_comma) states that an *omission* of a comma before the last term can suggest a stronger connection between the last two items in a series.
ykominami/d3,ykominami/d3