📝 readme: warn about modexp chain support

addresses quantstamp EXA-6

README.md

@@ -16,9 +16,13 @@ the account. Its core features include:
   - `upgradeToAndCall`
   - `execute` / `executeBatch`
 
-> [!CAUTION]
+> [!IMPORTANT]
 > The code in this repository and its dependencies are still under audit.
   It is not yet recommended for production use.
+>
+> FreshCryptoLib uses the `ModExp` precompile (`address(0x05)`), which is not supported
+> on some chains, such as [Polygon zkEVM](https://www.rollup.codes/polygon-zkevm#precompiled-contracts).
+> This plugin will not work on such chains, unless they support the RIP-7212 precompile.
 
 ## Developing