Add GoReportCard to main README

Author: goltermann

README.md

@@ -1,9 +1,11 @@
 # Kubernetes
 
-[![GoDoc Widget]][GoDoc] [![Travis Widget]][Travis] [![Coverage Status Widget]][Coverage Status]
+[![GoReportCard Widget]][GoReportCard] [![GoDoc Widget]][GoDoc] [![Travis Widget]][Travis] [![Coverage Status Widget]][Coverage Status]
 
 [GoDoc]: https://godoc.org/k8s.io/kubernetes
 [GoDoc Widget]: https://godoc.org/k8s.io/kubernetes?status.svg
+[GoReportCard]: https://goreportcard.com/report/k8s.io/kubernetes
+[GoReportCard Widget]: https://goreportcard.com/badge/k8s.io/kubernetes
 [Travis]: https://travis-ci.org/kubernetes/kubernetes
 [Travis Widget]: https://travis-ci.org/kubernetes/kubernetes.svg?branch=master
 [Coverage Status]: https://coveralls.io/r/kubernetes/kubernetes