readme: mock encoding docs

src/apps/mock/README.md

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+# Mock implementation
+
+This is a very simple mock implementation of how to verify messages. A designated signer is stored in the contract. If the address is signed by that address it is considered good.
+
+# Address encoding
+
+Mock uses 65 bytes to represent addresses. 64 for the address and 1 to describe the length. This is intended to be compatible with the vast majority of chains where 32 bytes might not.
+
+EVM -> EVM
+
+```solidity
+
+function _to_bytes65(address target) internal returns(bytes memory) {
+    return abi.encodePacked(
+        uint8(20),
+        bytes32(0),
+        uint256(uint160(target))
+    )
+}
+
+```