[doc] Add coverage badge to Readme

Author: g3force

README.md

@@ -2,6 +2,7 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/RoboCup-SSL/ssl-game-controller?style=flat-square)](https://goreportcard.com/report/github.com/RoboCup-SSL/ssl-game-controller)
 [![Go Doc](https://img.shields.io/badge/godoc-reference-blue.svg?style=flat-square)](https://godoc.org/github.com/RoboCup-SSL/ssl-game-controller/internal/app/controller)
 [![Release](https://img.shields.io/github/release/RoboCup-SSL/ssl-game-controller.svg?style=flat-square)](https://github.com/RoboCup-SSL/ssl-game-controller/releases/latest)
+[![Coverage](https://img.shields.io/badge/coverage-report-blue.svg)](https://circleci.com/api/v1.1/project/github/RoboCup-SSL/ssl-game-controller/latest/artifacts/0/coverage?branch=master)
 
 # ssl-game-controller