Mention Programmable HTTP Caching and Serving spec

[This spec](http://www.w3.org/TR/DataCache/), worked on by Nikunj Mehta in 2009/2010/2011, has much in common with NavigationController and is much more similar in design/goals/ideas than AppCache. Although the spec was abandoned, IMO it deserves a mention as a short historical note so that people can get to it from here. Making a pull request to get it added as [suggested on Twitter](https://twitter.com/slightlylate/status/344779565056528384).

Not sure if I put the note in the right place, but that's the only place that made sense to me. Will gladly edit and/or move it to another place if needed.