Add Maven badge

Author: mrotteveel

README.md

@@ -2,6 +2,7 @@ firebird-testcontainers-java
 ============================
 
 [![Build Status](https://travis-ci.com/FirebirdSQL/firebird-testcontainers-java.svg?branch=master)](https://travis-ci.com/FirebirdSQL/firebird-testcontainers-java)
+[![MavenCentral](https://maven-badges.herokuapp.com/maven-central/org.firebirdsql/firebird-testcontainers-java/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.firebirdsql/firebird-testcontainers-java/)
 
 Firebird-testcontainers-java is a module for [Testcontainers](https://www.testcontainers.org/)
 to provide lightweight, throwaway instances of Firebird for JUnit tests.