README: add the quay badge to the official git builds

README.md

@@ -1,6 +1,7 @@
 # etcd
 
 [![Build Status](https://travis-ci.org/coreos/etcd.png?branch=master)](https://travis-ci.org/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)
 
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