Document the "post-list" reST extension

I found the "post-list" reST extension that's bundled with Nikola tremendously useful. I'd say it deserves some documentation. Of course, you guys are busy with *actual code*, so I thought I'd write something myself. I hope you find it useful.

(Following up on an issue created [elsewhere](https://github.com/getnikola/nikola-site/issues/12))