Readme: added deprecated message

Author: pratikspatil024

README.md

@@ -1,3 +1,11 @@
+> **⚠️ This repository is archived and no longer maintained**
+>
+> This repository was used for the legacy Heimdall implementation on Polygon. Following the Heimdall migration, this codebase is now deprecated and will no longer receive updates or support.
+>
+> ➡️ **The new Heimdall implementation is available at:** [https://github.com/0xPolygon/heimdall-v2](https://github.com/0xPolygon/heimdall-v2)
+>
+> Please refer to the new repository for all future development, issues, and documentation.
+
 # Heimdall
 
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