add a token node to this document graph

def _add_token_to_document(self, token_string, token_attrs=None):
        """add a token node to this document graph"""
        token_feat = {self.ns+':token': token_string}
        if token_attrs:
            token_attrs.update(token_feat)
        else:
            token_attrs = token_feat
        token_id = 'token_{}'.format(self.token_count)
        self.add_node(token_id, layers={self.ns, self.ns+':token'},
                      attr_dict=token_attrs)
        self.token_count += 1
        self.tokens.append(token_id)
        return token_id