Update README.md

Author: wenbinf

README.md

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 # Podcast API Java Library
 
-[![Java CI](https://github.com/ListenNotes/podcast-api-java/actions/workflows/gradle.yml/badge.svg)](https://github.com/ListenNotes/podcast-api-java/actions/workflows/gradle.yml)
+[![Java CI](https://github.com/ListenNotes/podcast-api-java/actions/workflows/gradle.yml/badge.svg)](https://github.com/ListenNotes/podcast-api-java/actions/workflows/gradle.yml) [![Maven metadata URL](https://img.shields.io/maven-metadata/v?metadataUrl=https%3A%2F%2Frepo1.maven.org%2Fmaven2%2Fcom%2Flistennotes%2Fpodcast-api%2Fmaven-metadata.xml)](https://search.maven.org/artifact/com.listennotes/podcast-api)
 
 The Podcast API Java library provides convenient access to the [Listen Notes Podcast API](https://www.listennotes.com/podcast-api/) from
 applications written in JVM-based languages, including Java, Kotlin, Clojure, Scala, Groovy...