add maven badge

Author: alexcheng1982

README.md

@@ -1,5 +1,6 @@
 # ADK Extra
 
+![Maven Central Version](https://img.shields.io/maven-central/v/com.javaaidev.adk/adk-extra)
 [![build](https://github.com/JavaAIDev/adk-extra/actions/workflows/build.yaml/badge.svg)](https://github.com/JavaAIDev/adk-extra/actions/workflows/build.yaml)
 
 Extra Library for Google Agent Development Kit