Better instructions to configure preview

README.md

@@ -22,7 +22,10 @@ Make sure you have [MELPA](http://melpa.org/) enabled in your ``emacs.d``. Then,
     M-x package-install<RET>
     puml-mode<RET>
 
-Enjoy!
+Also, to enable preview you need to tell `puml-mode` where to locate the PlantUML JAR file. By default it will look for it in `~/plantuml.jar`, but you can specify any location with:
+
+    M-x customize-variable<RET>
+    puml-plantuml-jar-path
 
 Features
 ========