add Maven Central badge

Author: hboutemy

README.md

@@ -1,7 +1,8 @@
 # MojoHaus Properties Maven Plugin
 
 This is the [properties-maven-plugin](http://www.mojohaus.org/properties-maven-plugin/).
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+
+[![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.mojo/properties-maven-plugin.svg?label=Maven%20Central)](https://search.maven.org/artifact/org.codehaus.mojo/properties-maven-plugin)
 [![Build Status](https://travis-ci.org/mojohaus/properties-maven-plugin.svg?branch=master)](https://travis-ci.org/mojohaus/properties-maven-plugin)
 
 ## Releasing