message: add an example of new query funtion
diff: @@ -38,6 +38,9 @@ int main(int argc, char * argv[])
  std::string answer = gc.query("root", "123456", "test", sparql);
  std::cout << answer << std::endl;
  + // make a SPARQL query and save the result in ans.txt
  + gc.query("root", "123456", "test", sparql, "ans.txt");
  +
  // unload this database.
  gc.unload("test", "root", "123456");
  // also, you can load some exist database directly and then query.
  