docs: 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)