changed readme

Author: mpv1989

README.md

@@ -9,6 +9,8 @@
 4.1: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=4.1)](https://travis-ci.org/arangodb/arangodb-java-driver) 
 master: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=master)](https://travis-ci.org/arangodb/arangodb-java-driver)
 
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.arangodb/arangodb-java-driver/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.arangodb/arangodb-java-driver)
+
 ## Supported versions
 
 <table>