docs: add latest zenodo DOI to README

Author: jvwilliams23

README.md

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 # PYthon Statistical Shape and Appearance Modelling (pyssam)
 
+[![DOI](https://zenodo.org/badge/48445365.svg)](https://zenodo.org/badge/latestdoi/48445365)
+
 Welcome to **pyssam**, a Python library for statistical modelling of biomedical shape and appearance.