add maven central badge in README.md

Author: harkue

README.md

@@ -1,6 +1,8 @@
 java-xmlbuilder
 ===============
 
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.jamesmurty.utils/java-xmlbuilder/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.jamesmurty.utils/java-xmlbuilder)
+
 XML Builder is a utility that allows simple XML documents to be constructed
 using relatively sparse Java code.