chore (readme): add coveralls badge

README.md

@@ -2,6 +2,7 @@
 
 [![Join the chat at https://gitter.im/ovh-cds/Lobby](https://badges.gitter.im/ovh-cds/Lobby.svg)](https://gitter.im/ovh-cds/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Go Report Card](https://goreportcard.com/badge/github.com/ovh/cds)](https://goreportcard.com/report/github.com/ovh/cds)
+[![Coverage Status](https://coveralls.io/repos/github/ovh/cds/badge.svg?branch=master)](https://coveralls.io/github/ovh/cds?branch=master)
 
 <img align="right" src="https://raw.githubusercontent.com/ovh/cds/master/logo-background.png" width="25%">