Merge pull request #2090 from Adirio/add-coverage-badge

🌱 Add coverage badge from Coveralls

Author: k8s-ci-robot

README.md

@@ -1,5 +1,6 @@
 [![Build Status](https://travis-ci.org/kubernetes-sigs/kubebuilder.svg?branch=master)](https://travis-ci.org/kubernetes-sigs/kubebuilder "Travis")
 [![Go Report Card](https://goreportcard.com/badge/sigs.k8s.io/kubebuilder)](https://goreportcard.com/report/sigs.k8s.io/kubebuilder)
+[![Coverage Status](https://coveralls.io/repos/github/kubernetes-sigs/kubebuilder/badge.svg?branch=master)](https://coveralls.io/github/kubernetes-sigs/kubebuilder?branch=master)
 
 ## Kubebuilder