Update README.md

Added MavenCentral badge

Author: keilw

README.md

@@ -1,5 +1,6 @@
 unit-api
 ========
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/javax.measure/unit-api/badge.svg)](https://maven-badges.herokuapp.com/maven-central/javax.measure/unit-api)
 [![Build Status](https://drone.io/github.com/unitsofmeasurement/unit-api/status.png)](https://drone.io/github.com/unitsofmeasurement/unit-api/latest)
 [![Join the chat at https://gitter.im/unitsofmeasurement/unit-api](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/unitsofmeasurement/unit-api?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)