Added Zenodo DOI badge

Author: mmcdermott

README.md

@@ -9,6 +9,7 @@
 [![license](https://img.shields.io/badge/License-MIT-green.svg?labelColor=gray)](https://github.com/Medical-Event-Data-Standard/flexible_schema#license)
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+[![DOI](https://zenodo.org/badge/945606287.svg)](https://doi.org/10.5281/zenodo.17535818)
 
 `flexible_schema` provides a simple vehicle to specify and validate schemas for PyArrow tables and JSON
 objects that permit extension tables with additional columns, optional columns that may be missing