Added Badge (#93)

Added a badge

Author: PaulJonasJost

README.md

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+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.15355753.svg)](https://doi.org/10.5281/zenodo.15355753)
 # PEtabGUI
 
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 This is a graphical user interface to create parameter estimation problems. It is 
 based on the [PEtab](https://petab.readthedocs.io/en/latest/#) format.