Clarify role of MinimalForwarder (OpenZeppelin#3399)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/metatx/MinimalForwarder.sol

@@ -8,6 +8,11 @@ import "../utils/cryptography/draft-EIP712.sol";
 
 /**
  * @dev Simple minimal forwarder to be used together with an ERC2771 compatible contract. See {ERC2771Context}.
+ *
+ * MinimalForwarder is mainly meant for testing, as it is missing features to be a good production-ready forwarder. This
+ * contract does not intend to have all the properties that are needed for a sound forwarding system. A fully
+ * functioning forwarding system with good properties requires more complexity. We suggest you look at other projects
+ * such as the GSN which do have the goal of building a system like that.
  */
 contract MinimalForwarder is EIP712 {
     using ECDSA for bytes32;