add license badge

readme.md

@@ -2,6 +2,7 @@
 
 [![Go](https://github.com/tal-tech/go-zero/workflows/Go/badge.svg?branch=master)](https://github.com/tal-tech/go-zero/actions)
 [![codecov](https://codecov.io/gh/tal-tech/go-zero/branch/master/graph/badge.svg)](https://codecov.io/gh/tal-tech/go-zero)
+[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
 ## 1. go-zero框架背景