add license badge

README.md

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 # SFTPGo
-[![Build Status](https://travis-ci.org/drakkan/sftpgo.svg?branch=master)](https://travis-ci.org/drakkan/sftpgo) [![Code Coverage](https://codecov.io/gh/drakkan/sftpgo/branch/master/graph/badge.svg)](https://codecov.io/gh/drakkan/sftpgo/branch/master) [![Go Report Card](https://goreportcard.com/badge/github.com/drakkan/sftpgo)](https://goreportcard.com/report/github.com/drakkan/sftpgo) 
+[![Build Status](https://travis-ci.org/drakkan/sftpgo.svg?branch=master)](https://travis-ci.org/drakkan/sftpgo) [![Code Coverage](https://codecov.io/gh/drakkan/sftpgo/branch/master/graph/badge.svg)](https://codecov.io/gh/drakkan/sftpgo/branch/master) [![Go Report Card](https://goreportcard.com/badge/github.com/drakkan/sftpgo)](https://goreportcard.com/report/github.com/drakkan/sftpgo) [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 
 Full featured and highly configurable SFTP server software