[SC-452] Refactor forwarders and receivers

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "lib/forge-std"]
 	path = lib/forge-std
 	url = https://github.com/foundry-rs/forge-std
+[submodule "lib/openzeppelin-contracts"]
+	path = lib/openzeppelin-contracts
+	url = https://github.com/openzeppelin/openzeppelin-contracts

README.md

@@ -1,5 +1,22 @@
 # xchain-helpers
 
-This repository contains tooling for multichain testing such as cross-chain e2e testing support.
+This repository has three tools for use with multi-chain development. Domains refer to blockchains which are connected by bridges. Domains may have multiple bridges connecting them, for example both the Optimism Native Bridge and Circle CCTP connect Ethereum and Optimism domains.
+
+## Forwarders
+
+These libraries provide standardized syntax for sending a message to a bridge.
+
+## Receivers
+
+![xchain-helpers](.assets/xchain-helpers.png)
+
+The most common pattern is to have an authorized contract forward a message to another "business logic" contract to abstract away bridge dependencies. Receivers are contracts which perform this generic translation - decoding the bridge-specific message and forwarding to another `target` contract. The `target` contract should have logic to restrict who can call it and permission this to one or more bridge receivers.
+
+Most receivers implement a `fallback()` function which after validating that the call came from an authorized party on the other side of the bridge will forward the call to the `target` contract with the same function signature. This separation of concerns makes it easy for the receiver contract to focus on validating the bridge message, and the business logic `target` contract can validate the `msg.sender` comes from the receiver which validates the whole process. This ensures no chain-specific code is required for the business logic contract.
+
+## E2E Testing Infrastructure
+
+Provides tooling to record messages sent to supported bridges and relay them on the other side simulating a real message going across.
+
 ***
 *The IP in this repository was assigned to Mars SPC Limited in respect of the MarsOne SP*