Add maven badge

Author: acoburn

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/trellis-ldp/trellis-namespaces.png?branch=master)](https://travis-ci.org/trellis-ldp/trellis-namespaces)
 [![Build status](https://ci.appveyor.com/api/projects/status/ome8bbcjpveuq32n?svg=true)](https://ci.appveyor.com/project/acoburn/trellis-namespaces)
 [![Coverage Status](https://coveralls.io/repos/github/trellis-ldp/trellis-namespaces/badge.svg?branch=master)](https://coveralls.io/github/trellis-ldp/trellis-namespaces?branch=master)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.trellisldp/trellis-namespaces/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.trellisldp/trellis-namespaces/)
 
 A namespace service for a trellis repository.