chore: add DOI to readme

Author: sanjayankur31

README.md

@@ -11,6 +11,8 @@ pyNeuroML
 [![GitHub Org's stars](https://img.shields.io/github/stars/NeuroML?style=social)](https://github.com/NeuroML)
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+[![DOI](https://zenodo.org/badge/27832592.svg)](https://zenodo.org/doi/10.5281/zenodo.5744044)
+
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