Added link to Fredrik Lundh's online Tkinter reference material.

Doc/lib/libundoc.tex

@@ -15,7 +15,10 @@ effort spent.
 \begin{description}
 \item[Tkinter.py]
 --- Interface to Tcl/Tk for graphical user interfaces;
-Fredrik Lundh is working on this one!
+Fredrik Lundh is working on this one!  See \emph{An Introduction to
+Tkinter} at
+\url{http://www.pythonware.com/library/tkinter/introduction} for
+on-line reference material.
 
 \item[Tkdnd.py]
 --- Drag-and-drop support for \module{Tkinter}.