add doi to readme

README.md

 [![Build Status](https://travis-ci.org/amueller/word_cloud.png)](https://travis-ci.org/amueller/word_cloud)
 [![licence](http://img.shields.io/badge/licence-MIT-blue.svg?style=flat)](https://github.com/amueller/word_cloud/blob/master/LICENSE)
+[![DOI](https://zenodo.org/badge/21369/amueller/word_cloud.svg)](https://zenodo.org/badge/latestdoi/21369/amueller/word_cloud)
+
+
 
 word_cloud
 ==========