Update README.md

Author: robertoschwald

README.md

@@ -17,7 +17,8 @@ For older Grails versions, see "Supported Grails Versions" below.
  * For 1.x documentation, see [1.x Grails Plugin Page](http://grails.org/plugin/audit-logging "Grails Plugin Page")
 
 ## Supported Grails versions
- * Grails   3.3.x: [master branch](https://github.com/robertoschwald/grails-audit-logging-plugin/tree/master)
+ * Grails   4.0.x: [master_branch](https://github.com/robertoschwald/grails-audit-logging-plugin/tree/master)
+ * Grails   3.3.x: [3.x_maintenance_branch](https://github.com/robertoschwald/grails-audit-logging-plugin/tree/3.x_maintenance)
  * Grails   3.0.x-3.2.x: [2.x_maintenance branch](https://github.com/robertoschwald/grails-audit-logging-plugin/tree/2.x_maintenance)
  * Grails   2.x: [1.x_maintenance branch](https://github.com/robertoschwald/grails-audit-logging-plugin/tree/1.x_maintenance)