Add "DOI badge" to README page. Fix #1610 (#1639)

* Add "DOI badge" to gensim #1610

* reorder badges

README.md

@@ -3,14 +3,14 @@ gensim – Topic Modelling in Python
 
 [![Build Status](https://travis-ci.org/RaRe-Technologies/gensim.svg?branch=develop)](https://travis-ci.org/RaRe-Technologies/gensim)
 [![GitHub release](https://img.shields.io/github/release/rare-technologies/gensim.svg?maxAge=3600)](https://github.com/RaRe-Technologies/gensim/releases)
-[![Wheel](https://img.shields.io/pypi/wheel/gensim.svg)](https://pypi.python.org/pypi/gensim) 
+[![Wheel](https://img.shields.io/pypi/wheel/gensim.svg)](https://pypi.python.org/pypi/gensim)
+[![DOI](https://zenodo.org/badge/DOI/10.13140/2.1.2393.1847.svg)](https://doi.org/10.13140/2.1.2393.1847)
 [![Mailing List](https://img.shields.io/badge/-Mailing%20List-lightgrey.svg)](https://groups.google.com/forum/#!forum/gensim)
 [![Gitter](https://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-09a3d5.svg)](https://gitter.im/RaRe-Technologies/gensim)
 [![Follow](https://img.shields.io/twitter/follow/spacy_io.svg?style=social&label=Follow)](https://twitter.com/gensim_py)
 
 
 
-
 Gensim is a Python library for *topic modelling*, *document indexing*
 and *similarity retrieval* with large corpora. Target audience is the
 *natural language processing* (NLP) and *information retrieval* (IR)