zmq4: add DOI badge

Author: sbinet

README.md

@@ -3,7 +3,7 @@
 [![Build Status](https://travis-ci.org/go-zeromq/zmq4.svg?branch=master)](https://travis-ci.org/go-zeromq/zmq4)
 [![codecov](https://codecov.io/gh/go-zeromq/zmq4/branch/master/graph/badge.svg)](https://codecov.io/gh/go-zeromq/zmq4)
 [![GoDoc](https://godoc.org/github.com/go-zeromq/zmq4?status.svg)](https://godoc.org/github.com/go-zeromq/zmq4)
-
+[![DOI](https://zenodo.org/badge/129430151.svg)](https://zenodo.org/badge/latestdoi/129430151)
 
 `zmq4` is a pure-Go implementation of ØMQ (ZeroMQ), version 4.