Twitter Bootstrap logo added

README.md

-Twitter Bootstrap Extension for Yii 2
-=====================================
+<p align="center">
+    <a href="http://getbootstrap.com/" target="_blank" rel="external">
+        <img src="https://v4-alpha.getbootstrap.com/assets/brand/bootstrap-solid.svg" height="80px">
+    </a>
+    <h1 align="center">Twitter Bootstrap Extension for Yii 2</h1>
+    <br>
+</p>
 
 This is the Twitter Bootstrap extension for [Yii framework 2.0](http://www.yiiframework.com). It encapsulates [Bootstrap](http://getbootstrap.com/) components
 and plugins in terms of Yii widgets, and thus makes using Bootstrap components/plugins