Added warning in README, as fix to issue 24 (#25)

README.md

@@ -1,3 +1,7 @@
+> [!WARNING]
+> As of May 2024, this repository is no longer actively maintained. All code has been directly integrated into [MQT Qudits](https://github.com/cda-tum/mqt-qudits).
+> Development is expected to continue there. No new contributions will be accepted here.
+
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