Update README.md

add Zenodo badge and recommended citation

README.md

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 **py**thon **PR**eprocessing of **I**nputs for **M**odel fr**A**meworks: a tool to automate the creation of energy system models using a common database
 
 [![Documentation Status](https://readthedocs.org/projects/pyprima/badge/?version=latest)](http://pyprima.readthedocs.io/en/latest/?badge=latest)
+[![DOI](https://zenodo.org/badge/186612855.svg)](https://zenodo.org/badge/latestdoi/186612855)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
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@@ -35,3 +36,6 @@ Thanks goes to these wonderful people ([emoji key](https://allcontributors.org/d
 <!-- ALL-CONTRIBUTORS-LIST:END -->
 
 This project follows the [all-contributors](https://github.com/all-contributors/all-contributors) specification. Contributions of any kind welcome!
+
+## Please cite as:
+Kais Siala, & Houssame Houmy. (2020, June 1). tum-ens/pyPRIMA: python PReprocessing of Inputs for Model frAmeworks (Version v1.0.0). Zenodo. http://doi.org/10.5281/zenodo.3872023