Added Maven badges

Author: devgianlu

README.md

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 # librespot-java
 [![Build Status](https://travis-ci.org/librespot-org/librespot-java.svg?branch=master)](https://travis-ci.org/librespot-org/librespot-java)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-java/badge.svg)](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-java)
 
 `librespot-java` is a port of [librespot](https://github.com/librespot-org/librespot), originally written in Rust. Additionally, this implementation provides an useful API to request metadata or control the player, more [here](https://github.com/librespot-org/librespot-java/blob/master/api).
 

api-client/README.md

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 # librespot-api-client
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-api-client/badge.svg)](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-api-client)
+
 A GUI client for the [librespot-api](https://github.com/librespot-org/librespot-java/tree/master/api) made with JavaFX.

api/README.md

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 # librespot-api
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-api/badge.svg)](https://maven-badges.herokuapp.com/maven-central/xyz.gianlu.librespot/librespot-api)
+
 This module depends on `librespot-core` and provides an API to interact with the Spotify client.
 
 ## How it works