Add deprecation notice

Author: Nashtare

README.md

@@ -3,6 +3,11 @@
 
 This repository was originally for Plonky2, a SNARK implementation based on techniques from PLONK and FRI. It has since expanded to include tools such as Starky, a highly performant STARK implementation.
 
+## ⚠️ Plonky2 Deprecation Notice
+
+Plonky2 is being deprecated and will no longer receive updates or support.
+
+Please consider using **[Plonky3](https://github.com/Plonky3/Plonky3)** instead, Polygon's next-generation ZK proving system.
 
 ## Documentation