Added logo to readme [skip ci]

Author: samdark

README.md

@@ -1,5 +1,10 @@
-API documentation generator for Yii 2
-=====================================
+<p align="center">
+    <a href="https://github.com/yiisoft" target="_blank">
+        <img src="https://avatars0.githubusercontent.com/u/993323" height="100px">
+    </a>
+    <h1 align="center">API documentation generator for Yii 2</h1>
+    <br>
+</p>
 
 This extension provides an API documentation generator for the [Yii framework 2.0](http://www.yiiframework.com).