Mention interactive demo in readme

I've put together a [little demo](https://github.com/tlvince/dox-foundation-demo) to see how dox-foundation runs for real.

Note it highlights a couple of potential issues:

* An `overflow: ...` rule could be set on the TOC sections
* A base dir option (or equivalent) could be added to mitigate [redirects](https://github.com/tlvince/dox-foundation-demo/blob/master/server.js#L12-L14)