add fee requirement

Author: godzillaba

contracts/tokenbridge/libraries/vault/MasterVault.sol

@@ -23,6 +23,7 @@ import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol
 ///         For a subVault to be compatible with the MasterVault, it must adhere to the following:
 ///         - It must be able to handle arbitrarily large deposits and withdrawals
 ///         - Deposit size or withdrawal size must not affect the exchange rate (i.e. no slippage)
+///         - Must not have deposit or withdrawal fees (or the deposit/withdrawal fee beneficiary is trusted by all MasterVault users)
 ///
 ///         For performance fees to be enabled, the subVault should also have a manipulation resistant 
 ///         convertToAssets function. If convertToAssets can be manipulated,