chore: adding predeployed address to be defined

Author: 0xng

specs/protocol/rollback-inboxes.md

@@ -14,6 +14,8 @@
 
 The rollback inbox contracts are responsible for storing the hashes of messages that failed on the other domain. They are built with the purpose of allowing contracts on the same domain to have the capability of handling what actions to perform when a message failed on the other domain and was rolled-back to it's origin domain.
 
+The `L2RollbackInbox` is a predeploy contract located at TO BE DEFINED.
+
 ```solidity
 interface RollbackInbox {
     event MessageHashReceived(bytes32 indexed messageHash, uint256 indexed timestamp);