Add Maven Central badge

Author: gaul

README.md

@@ -1,5 +1,7 @@
 # Java httpbin
 
+[![Maven Central](https://img.shields.io/maven-central/v/org.gaul/httpbin.svg)](https://search.maven.org/#search%7Cga%7C1%7Ca%3A%22httpbin%22)
+
 A Java-based HTTP server that lets you test your HTTP client, retry logic,
 streaming behavior, timeouts etc. with the endpoints of
 [httpbin.org](https://httpbin.org/) locally.