README: add License shield (#8235)

README.md

 [![GitHub release](https://img.shields.io/github/release/go-gitea/gitea.svg)](https://github.com/go-gitea/gitea/releases/latest)
 [![Help Contribute to Open Source](https://www.codetriage.com/go-gitea/gitea/badges/users.svg)](https://www.codetriage.com/go-gitea/gitea)
 [![Become a backer/sponsor of gitea](https://opencollective.com/gitea/tiers/backers/badge.svg?label=backer&color=brightgreen)](https://opencollective.com/gitea)
+[![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 
 ## Purpose
 

README_ZH.md

 [![GoDoc](https://godoc.org/code.gitea.io/gitea?status.svg)](https://godoc.org/code.gitea.io/gitea)
 [![GitHub release](https://img.shields.io/github/release/go-gitea/gitea.svg)](https://github.com/go-gitea/gitea/releases/latest)
 [![Become a backer/sponsor of gitea](https://opencollective.com/gitea/tiers/backers/badge.svg?label=backer&color=brightgreen)](https://opencollective.com/gitea)
+[![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 
 ## 目标