Add link to Atom feed

I also changed the feed's file name to clue GitHub into using correct HTTP headers ([reference](http://taylor.fausak.me/2012/04/26/serving-atom-feeds-with-github-pages/)).

The Communications group today recommended registering the feed with Feedburner so that people can subscribe to the feed by email.

Once the file name is changed, we can:

* [ ] Submit the feed to Feedburner
* [ ] Update the `<link>` tag to point to the new feed URL
* [ ] Add a Feedburner widget somewhere for subscribing via email