Docs: List unexpected-preact on the plugins page

documentation/plugins.md

@@ -45,6 +45,7 @@ Here's a partial list of plugins for Unexpected:
   Only works with node.js and io.js.
 * [unexpected-moment](http://unexpected.js.org/unexpected-moment/):
   Assertions for testing [moment.js](http://momentjs.com/) instances.
+* [unexpected-preact](https://bruderstein.github.io/unexpected-preact/): Assertions for preact.js
 * [unexpected-react](http://bruderstein.github.io/unexpected-react): Assertions for React.js. Assert using JSX.
 * [unexpected-reaction](http://unexpected.js.org/unexpected-reaction/): An Unexpected plugin to make React testing with [unexpected-dom](https://munter.github.io/unexpected-dom/) more convenient.
 * [unexpected-resemble](http://unexpected.js.org/unexpected-resemble/): Image resemblance