Add example for how to lock balance on the contract

[Kaladin13]

TACT function where the user sends amount_sent and specifies amount_to_lock. The contract should:

Lock exactly amount_to_lock in state
Refund the surplus (amount_sent – amount_to_lock) to the sender after covering all fees (incl. storage rent)

Reference:

      nativeReserve(msg.amount, ReserveExact | ReserveAddOriginalBalance | ReserveBounceIfActionFail);

      message(MessageParameters {
                to: sender(),
                value: 0,
                bounce: false,
                mode: SendRemainingBalance,
            });