+ jsDelvr CDN mention

Added mention for jsDelivr free [CDN](http://www.jsdelivr.com/about.php).
Everytime you push a new [release](https://github.com/scottjehl/picturefill/releases), jsDelivr will be automatically updated.

At first I thought about adding a HTML code example, but figured adding a side-note under 'advanced' usage is keep with tone?