Update README.md

Author: Zenlua

README.md

@@ -3,6 +3,7 @@
 
 [![Release](https://img.shields.io/github/v/release/Zenlua/Tool-Tree?label=Release)](https://github.com/Zenlua/Tool-Tree/releases/latest)
 [![GitHub](https://img.shields.io/github/stars/Zenlua/Tool-Tree?style=flat)](https://github.com/Zenlua/Tool-Tree)
+[![License](https://img.shields.io/github/license/Zenlua/Tool-Tree)](https://github.com/Zenlua/Tool-Tree)
 
 Official website: **https://zenlua.github.io/Tool-Tree**