Add maven central version badge

README.md

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 ![KryoNet](https://raw.github.com/wiki/EsotericSoftware/kryo/images/logo.jpg)
 
 [![Build Status](https://jenkins.inoio.de/buildStatus/icon?job=kryo&foo=bar)](https://jenkins.inoio.de/job/kryo/)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.esotericsoftware/kryo/badge.svg)](http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.esotericsoftware%22%20AND%20a%3Akryo)
 [![Join the chat at https://gitter.im/EsotericSoftware/kryo](https://badges.gitter.im/EsotericSoftware/kryo.svg)](https://gitter.im/EsotericSoftware/kryo)
 
 Kryo is a fast and efficient object graph serialization framework for Java. The goals of the project are speed, efficiency, and an easy to use API. The project is useful any time objects need to be persisted, whether to a file, database, or over the network.