Added total downloads badge to README.md

Author: creocoder

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://img.shields.io/travis/creocoder/yii2-nested-sets/master.svg?style=flat-square)](https://travis-ci.org/creocoder/yii2-nested-sets)
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 A modern nested sets behavior for the Yii framework utilizing the Modified Preorder Tree Traversal algorithm.