Add Maven Central badge to README

Author: mpkorstanje

README.md

@@ -1,5 +1,7 @@
 ⚠️ This is an internal package; you don't need to install it in order to use the JSON Formatter.
 
+[![Maven Central](https://img.shields.io/maven-central/v/io.cucumber/cucumber-json-formatter.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:io.cucumber%20AND%20a:cucumber-json-formatter)
+
 # Cucumber JSON Formatter
 Writes Cucumber message into the legacy Cucumber JSON format