[doc] Add Zenodo DOI badge to README

Use the concept DOI, which always resolves to the latest archived
release, so the badge does not need updating on every release.

Assisted-by: Claude <noreply@anthropic.com>

Author: Wentzell

README.md

@@ -1,3 +1,5 @@
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.21691748.svg)](https://doi.org/10.5281/zenodo.21691748)
+
 DFT_Tools - A TRIQS application for ab initio calculations
 
 Copyright (C) 2011-2019: M. Aichhorn, L. Pourovskii, V. Vildosola and C. Martins