Add a label on the document.

def __set_labels(self, labels):
        """
        Add a label on the document.
        """
        with self.fs.open(self.fs.join(self.path, self.LABEL_FILE), 'w') \
                as file_desc:
            for label in labels:
                file_desc.write("%s,%s\n" % (label.name,
                                             label.get_color_str()))