Update example.cpp

include license information in file

example/example.cpp

@@ -1,6 +1,20 @@
-//
-// Created by Fanzhe on 5/29/2017.
-//
+/* example.cpp
+ *
+ * Copyright (C) 20017-2021 Fanzhe Lyu <lvfanzhe@hotmail.com>, all rights reserved.
+ *
+ * modbuspp is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
 
 #include "modbus.h"