Add VFS to Testing.

README.md

 * [Mink](http://mink.behat.org/) - Web acceptance testing.
 * [HTTP Mock](https://github.com/InterNations/http-mock) - A library for mocking HTTP requests in unit tests.
 * [VFS Stream](https://github.com/mikey179/vfsStream) - A virtual filesystem stream wrapper for testing.
+* [VFS](https://github.com/adlawson/vfs.php) - Another virtual filesystem for testing.
 * [Locust](http://locust.io/) - A modern load test library written in Python.
 * [Travis CI](https://travis-ci.org/) - A continuous integration platform.
 * [PHPCI](http://www.phptesting.org/) - An open source continuous integration platform for PHP.