add hrule description

Docs/README.gui

@@ -390,6 +390,7 @@ Example:
    <max>360</max>
   </dial>
 
+
 textbox
 -------
 
@@ -413,4 +414,20 @@ property tree, like:
     <editable>true</editable> <!-- whether the puLargeInput is supposed to be editable -->
 </textbox>  
 
+
+hrule
+-----
+
+Draws a horizontal line. Accepts color and thickness (<pref-height>):
+
+  <hrule>
+    <color>
+     <red>1.0</red>
+     <green>0.0</green>
+     <blue>0.0</blue>
+    </color>
+    <pref-height>2</pref-height>
+   </hrule>
+
+
 __end__