Add maven badge

Author: britter

README.md

@@ -1,6 +1,7 @@
 spring-boot-admin
 =================
 [![Build Status](https://travis-ci.org/codecentric/spring-boot-admin.png?branch=master)](https://travis-ci.org/codecentric/spring-boot-admin)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/de.codecentric/spring-boot-admin/badge.svg)](https://maven-badges.herokuapp.com/maven-central/de.codecentric/spring-boot-admin/)
 [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/codecentric/spring-boot-admin?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 This is a simple admin interface for [Spring Boot](http://projects.spring.io/spring-boot/ "Official Spring-Boot website") applications.