Add Zenodo badge to README (#389)

Author: jukent

README.md

@@ -2,6 +2,8 @@
 
 [![nightly-build](https://github.com/ProjectPythia/pythia-foundations/actions/workflows/nightly-build.yaml/badge.svg)](https://github.com/ProjectPythia/pythia-foundations/actions/workflows/nightly-build.yaml)
 
+[![DOI](https://zenodo.org/badge/338145160.svg)](https://zenodo.org/badge/latestdoi/338145160)
+
 This is the source repository for the Pythia Foundations content collection.
 
 The rendered site can be found at https://foundations.projectpythia.org