adds DOI badge to readme

Author: Daniel Ruprecht

README.md

@@ -16,6 +16,8 @@ If you use it (or parts of it) for a publication, please cite
 
 (this will be updated once published).
 
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.228104.svg)](https://doi.org/10.5281/zenodo.228104)
+
 How can I reproduce Figures from the publication?
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