Add code quality report

Author: andig

README.md

@@ -1,6 +1,6 @@
 # evcc
 
-[![Build Status](https://travis-ci.org/andig/evcc.svg?branch=master)](https://travis-ci.org/andig/evcc)
+[![Build Status](https://travis-ci.org/andig/evcc.svg?branch=master)](https://travis-ci.org/andig/evcc) [![Code Quality](https://goreportcard.com/badge/github.com/andig/evcc)](https://goreportcard.com/report/github.com/andig/evcc)
 
 EVCC is an extensible EV Charge Controller with PV integration implemented in [Go](2).