Add an example for accordion on both small and large screens

If you want accordion functionality on both small and large screens, without tabs, the `accordion` class is required. Without the accordion class, the [javascript](https://github.com/zurb/foundation/blob/master/js/foundation/foundation.section.js#L138) will add top padding to the section element.

![when-no-accordion-class](https://f.cloud.github.com/assets/379519/257164/1a150120-8c49-11e2-9c32-ae671003fdc6.png)