[ADD]添加许可说明

README.md

@@ -62,3 +62,8 @@ web
 ├── latest_log  -- vue-cli 配置
 └── main.go  -- package.json
 ```
+#### License
+```dx
+MegEngine is Licensed under the Apache License, Version 2.0
+Copyright (c) 2014-2021 Megvii Inc. All rights reserved.
+```