Mention the CSS compilation bug in Assetpack

- If any css element contains a url link declaration to a non-standard
HTTP URI, Sinatra-Assetpack crashes and the css file will not get
loaded. There is a PR pending for this issue at
https://github.com/rstacruz/sinatra-assetpack/pull/152.

- Foundation's 'form.scss' component uses an SVG background image that
is loaded via the declaration `background-image: url('...')` where the
URI is a data uri. [http://css-tricks.com/using-svg/) ,
http://css-tricks.com/data-uris/].

- Hence, the example shows loading only the JS files via Assetpack. The
CSS file is linked directly in the HTML without any further processing.