Add a private note.

        Args:
            :param comment: comment about the author.
            :type comment: string

            :param source: the source of the comment.
            :type source: string

def add_private_note(self, note, source=None):
        """Add a private note.

        Args:
            :param comment: comment about the author.
            :type comment: string

            :param source: the source of the comment.
            :type source: string
        """
        note = {
            'value': note,
        }
        if source:
            note['source'] = source
        self._append_to('_private_notes', note)