Readme: adding gocard

Gocard includes most of the go style checking tools that we care about. This is a good indicator for us.

README.md

@@ -1,5 +1,6 @@
 # etcd
 
+[![Go Report Card](https://goreportcard.com/badge/github.com/coreos/etcd)](https://goreportcard.com/report/github.com/coreos/etcd)
 [![Build Status](https://travis-ci.org/coreos/etcd.svg?branch=master)](https://travis-ci.org/coreos/etcd)
 [![Build Status](https://semaphoreci.com/api/v1/projects/406f9909-2f4f-4839-b59e-95082cb088f1/575109/badge.svg)](https://semaphoreci.com/coreos/etcd)
 [![Docker Repository on Quay.io](https://quay.io/repository/coreos/etcd-git/status "Docker Repository on Quay.io")](https://quay.io/repository/coreos/etcd-git)