Add Zenodo badge
[![Windows Build status](https://ci.appveyor.com/api/projects/status/70knpvcgvmah2qin?svg=true)](https://ci.appveyor.com/project/jchodera/openmmtools)
[![Anaconda Badge](https://anaconda.org/omnia/openmmtools/badges/version.svg)](https://anaconda.org/omnia/openmmtools)
[![Downloads Badge](https://anaconda.org/omnia/openmmtools/badges/downloads.svg)](https://anaconda.org/omnia/openmmtools/files)
-[![Documentation Status](https://readthedocs.org/projects/openmmtools/badge/?version=latest)](http://openmmtools.readthedocs.io/en/latest/?badge=latest)
+[![ReadTheDocs Badge](https://readthedocs.org/projects/openmmtools/badge/?version=latest)](http://openmmtools.readthedocs.io/en/latest/?badge=latest)
+[![Zenodo DOI Badge](https://zenodo.org/badge/25416166.svg)](https://zenodo.org/badge/latestdoi/25416166)
# Various Python tools for OpenMM
