[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

@@ -1,4 +1,5 @@
 [![build](https://github.com/TRIQS/maxent/workflows/build/badge.svg)](https://github.com/TRIQS/maxent/actions?query=workflow%3Abuild)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.21691474.svg)](https://doi.org/10.5281/zenodo.21691474)
 
 # MaxEnt