Add README docs (NOTICE) for LinkTokenChild contract

Author: krebernisak

contracts/v0.7/bridge/README.md

@@ -1,3 +1,25 @@
 # LINK Token Bridge v0.7
 
-- `./token/LinkTokenChild.sol`: A mintable & burnable child LinkToken contract to be used on child networks.
+- `./token/LinkTokenChild.sol`: A generalized mintable & burnable child LinkToken contract to be used on child networks.
+
+NOTICE: Current implementation of LinkTokenChild contract requires some additional consideration:
+
+- Supporting more than one gateway (multiple bridges minting the same token) leaves room for accounting issues.
+  If we have more than one gateway supported, an additional check needs to exist that limits withdrawals per
+  gateway to an amount locked on L1, for the specific gateway. Otherwise one can accidentally "burn" tokens
+  by withdrawing more than locked in L1 (tx will fail on L1). When there is a 1:1 relationship between the
+  gateway and token, the token itself is an accounting mechanism. For a potential N:1 relationship, a more
+  sophisticated type of accounting needs to exist.
+- Every bridge is unique in the amount of risk it bears, so tokens bridged by different bridges are not 1:1
+  the same token, and shouldn't be forced as such.
+- Bridges often require an unique interface to be supported by the child network tokens
+  (e.g. `mint` vs. `deposit`, `burn` vs. `withdraw/unwrap`, etc.).
+- Bridges often assume that the child contract they are bridging to is the ERC20 token itself, not a gateway
+  (intermediate contract) that could help us map from the specific bridge interface to our standard
+  LinkTokenChild interface.
+- Chainlink often needs to launch on a new network before the native bridge interface is defined.
+- To support early (before the bridge is defined) Chainlink network launch, we could make an upgradeable
+  LinkTokenChild contract which would enable us to slightly update the contract interface after the bridge
+  gets defined, and once online transfer the ownership (bridge gateway role) to the new bridge.
+
+TODO: Potentially create an upgradeable `LinkTokenChild.sol` and limit gateway support to only one (owner)!

contracts/v0.7/bridge/token/LinkTokenChild.sol

@@ -11,17 +11,8 @@ import { ERC20Burnable } from "../../../../vendor/OpenZeppelin/openzeppelin-cont
 import { SimpleWriteAccessController } from "../../../../vendor/smartcontractkit/chainlink/evm-contracts/src/v0.6/SimpleWriteAccessController.sol";
 import { LinkToken } from "../../../v0.6/LinkToken.sol";
 
-/// @dev Access controlled mintable & burnable LinkToken, for use on sidechains and L2 networks.
+/// @dev  A generalized mintable & burnable child LinkToken contract to be used on child networks.
 contract LinkTokenChild is ITypeAndVersion, IERC20Child, SimpleWriteAccessController, ERC20Burnable, LinkToken {
-  /**
-   * @dev Overrides parent contract so no tokens are minted on deployment.
-   * @inheritdoc LinkToken
-   */
-  function _onCreate()
-    internal
-    override
-  {}
-
   /**
    * @notice versions:
    *
@@ -86,6 +77,15 @@ contract LinkTokenChild is ITypeAndVersion, IERC20Child, SimpleWriteAccessContro
     super.burnFrom(account, amount);
   }
 
+  /**
+   * @dev Overrides parent contract so no tokens are minted on deployment.
+   * @inheritdoc LinkToken
+   */
+  function _onCreate()
+    internal
+    override
+  {}
+
   /// @inheritdoc LinkToken
   function _transfer(
     address sender,
@@ -111,4 +111,4 @@ contract LinkTokenChild is ITypeAndVersion, IERC20Child, SimpleWriteAccessContro
   {
     super._approve(owner, spender, amount);
   }
-}
+}