Added Yii2 framework

README.md

 * [Zend Framework 2](http://framework.zend.com) - Another framework comprised of individual components.
 * [Laravel 4](http://laravel.com/) - A simple PHP framework.
 * [Aura PHP](http://auraphp.com/) - A framework of independent components.
+* [Yii2](https://github.com/yiisoft/yii2/) - a high-performance framework.
 * [Phalcon](http://phalconphp.com/en/) - A framework implemented as a C extension.
 
 ## Framework Extras