Merge pull request #18 from base-org/wilson/update-readme

add warning

README.md

@@ -4,6 +4,9 @@ Webauthn-sol is a Solidity library for verifying WebAuthn authentication asserti
 
 This library is optimized for Ethereum layer 2 rollup chains but will work on all EVM chains. Signature verification always attempts to use the [RIP-7212 precompile](https://github.com/ethereum/RIPs/blob/master/RIPS/rip-7212.md) and, if this fails, falls back to using [FreshCryptoLib](https://github.com/rdubois-crypto/FreshCryptoLib/blob/master/solidity/src/FCL_ecdsa.sol#L40).
 
+> [!IMPORTANT]  
+> 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 library will not work on such chains, unless they support the RIP-7212 precompile. 
+
 Code excerpts
 
 ```solidity