Add a note to the xml. If update param is None, it will be the current time.

def add_note(self, player, text, label=None, update=None):
        """Add a note to the xml. If update param is None, it will be the current time."""
        if label is not None and (label not in self.label_names):
            raise LabelNotFoundError('Invalid label: {}'.format(label))
        if update is None:
            update = datetime.utcnow()
        # converted to timestamp, rounded to ones
        update = update.strftime('%s')
        label_id = self._get_label_id(label)
        new_note = etree.Element('note', player=player, label=label_id, update=update)
        new_note.text = text
        self.root.append(new_note)