Add Maven Central badge to README.md

Author: bdemers

README.md

@@ -1,6 +1,7 @@
 [<img src="http://shiro.apache.org/assets/images/apache-shiro-logo.png" align="right" />](http://shiro.apache.org)
 
 [![Build Status](https://builds.apache.org/job/Shiro/badge/icon)](https://builds.apache.org/job/Shiro)
+[![Maven Central](https://img.shields.io/maven-central/v/org.apache.shiro/shiro-core.svg)]()
 
 Apache Shiro
 ============