Document QUnit.push

[Since this functionality is backed with unit tests](https://github.com/jquery/qunit/blob/81d499b0fb06970b5f52185b8153416341c0d49e/test/test.js#L630-L640), I thought it could use some documentation.

This commit adds a new category, so I expect this will also require a change to [the qunit.js site itself](https://github.com/jquery/qunitjs.com), but I'm not really sure how to orchestrate that. Alternatively, we could add this new entry to the "Configuration" category (although I would argue the discoverability of that approach).

Issue #17 sounds like it could be related. @jzaefferer: are "addons" equivalent to custom assertions?