chore(protocol-contracts): add mainnet registry address in doc (#1780)

Author: melanciani

protocol-contracts/confidential-token-wrappers-registry/docs/README.md

@@ -12,6 +12,10 @@ The **Confidential Token Wrappers Registry** is an on-chain directory that maps
 - **Invalid**: An invalid confidential wrapper has been revoked by the registry owner and should not be used to wrap and unwrap tokens from the underlying token.
 - **Owner**: The owner of the registry. In the FHEVM protocol, this is a DAO governance contract handled by Zama.
 
+## Mainnet address
+
+The official registry address on Ethereum mainnet is [`0xeb5015fF021DB115aCe010f23F55C2591059bBA0`](https://etherscan.io/address/0xeb5015fF021DB115aCe010f23F55C2591059bBA0).
+
 ## Quick Start
 
 > A token can only be associated with one confidential wrapper. A confidential wrapper can only be associated with one token.