Fixed broken link, added Admonition on adding niftygui library.

src/docs/asciidoc/jme3/advanced/nifty_gui_overlay.adoc

@@ -5,6 +5,7 @@
 :keywords: gui, documentation, nifty, hud
 :relfileprefix: ../../
 :imagesdir: ../..
+:experimental:
 ifdef::env-github,env-browser[:outfilesuffix: .adoc]
 
 
@@ -26,7 +27,13 @@ On this page, we look at the overlay variant, which is more commonly used in gam
 
 == Sample Code
 
-*  link:http://code.google.com/p/jmonkeyengine/source/browse/trunk/engine/src/test/jme3test/niftygui/TestNiftyGui.java[TestNiftyGui.java]
+
+*  link:https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-examples/src/main/java/jme3test/niftygui/TestNiftyGui.java[TestNiftyGui.java]
+
+[TIP]
+====
+The jme3-niftygui library is included in jMonkeyEngine. If you installed jMonkeyEngine using one of the <<wiki/#install#,optional methods>>, it will be added to your projects Library folder as part of the installation. If you're using the jMonkeyEngine SDK, you add it to any project by btn:[RMB] selecting your projects `Library` folder, choosing menu:Add Library[jme-niftygui] followed by `Add Library`.
+====
 
 
 == Overlaying the User Interface Over the Screen