Added link to forum topic for lemur

docs/modules/contributions/pages/gui/topic_contributions.adoc

@@ -0,0 +1,6 @@
+= Enter Document Title Here
+mitm001
+:description: GUI contributed libraries for the jmonkey engine.
+:keywords: gui, documentation, input, control, hud, contributions
+
+The official engine GUI library is xref:core:Nifty Gui.