TODO add docstring

def add_broadcast_message(self, rawtx, message, sender_wif,
                              dust_limit=common.DUST_LIMIT):
        """TODO add docstring"""
        tx = deserialize.tx(rawtx)
        message = deserialize.unicode_str(message)
        sender_key = deserialize.key(self.testnet, sender_wif)
        tx = control.add_broadcast_message(self.testnet, tx, message,
                                           sender_key, dust_limit=dust_limit)
        return serialize.tx(tx)