Update README.md

Co-authored-by: Hendrik Ranocha <[email protected]>

README.md

@@ -10,7 +10,7 @@
 [![Coveralls](https://coveralls.io/repos/github/trixi-framework/TrixiAtmo.jl/badge.svg?branch=main)](https://coveralls.io/github/trixi-framework/TrixiAtmo.jl?branch=main)
 [![License: MIT](https://img.shields.io/badge/License-MIT-success.svg)](https://opensource.org/licenses/MIT)
 <!--
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3996439.svg)](https://doi.org/10.5281/zenodo.3996439)
+[![DOI](https://zenodo.org/badge/DOI/TODO.svg)](https://doi.org/TODO)
 -->
 
 **Note: This repository is still in its alpha stage and anything might change at