add license

Author: Wang Allen

README.md

@@ -167,7 +167,14 @@ SkipList还有一个优势就是实现简单，SkipList的实现只花了2个小
     };
 
 ```
+## License
+Copyright 2018 AllenWang
 
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at
+```
+   http://www.apache.org/licenses/LICENSE-2.0
+```
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.