add maven-central badge (#616)

Author: bwaldvogel

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/JSQLParser/JSqlParser.svg?branch=master)](https://travis-ci.org/JSQLParser/JSqlParser)   [![Coverage Status](https://coveralls.io/repos/JSQLParser/JSqlParser/badge.svg?branch=master)](https://coveralls.io/r/JSQLParser/JSqlParser?branch=master)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/00b2d91995764ae4805b55627aca8d39)](https://www.codacy.com/app/wumpz/JSqlParser?utm_source=github.com&utm_medium=referral&utm_content=JSQLParser/JSqlParser&utm_campaign=Badge_Grade)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.github.jsqlparser/jsqlparser/badge.svg)](http://maven-badges.herokuapp.com/maven-central/com.github.jsqlparser/jsqlparser)
 
 [![PayPal donate button](http://img.shields.io/paypal/donate.png?color=blue)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=64CCN9JJANZXA "Help this JSqlParser version using Paypal")