add download badge

Author: josesho

README.md

@@ -2,6 +2,7 @@
 [![Travis CI build status](https://travis-ci.org/ACCLAB/DABEST-python.svg?branch=master)](https://travis-ci.org/ACCLAB/DABEST-python)
 [![minimal Python version](https://img.shields.io/badge/Python%3E%3D-3.5-6666ff.svg)](https://www.anaconda.com/distribution/)
 [![PyPI version](https://badge.fury.io/py/dabest.svg)](https://badge.fury.io/py/dabest)
+[![PyPI Downloads](https://img.shields.io/pypi/dm/dabest?color=bright-green)](https://pypistats.org/packages/dabest)
 [![Free-to-view citation](https://zenodo.org/badge/DOI/10.1038/s41592-019-0470-3.svg)](https://rdcu.be/bHhJ4)
 [![License](https://img.shields.io/badge/License-BSD%203--Clause--Clear-orange.svg)](https://spdx.org/licenses/BSD-3-Clause-Clear.html)