add code coverage badge

README.md

@@ -1,5 +1,5 @@
 # SFTPGo
-[![Build Status](https://travis-ci.org/drakkan/sftpgo.svg?branch=master)](https://travis-ci.org/drakkan/sftpgo) [![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) 
 
 Full featured and highly configurable SFTP server software