Clarify where "Documenting Python" can be found.

Doc/README.txt

@@ -7,7 +7,8 @@ available at http://docs.python.org/download/.
 
 Documentation on the authoring Python documentation, including information about
 both style and markup, is available in the "Documenting Python" chapter of the
-documentation.  There's also a chapter intended to point out differences to
+developers guide (http://docs.python.org/devguide/documenting.html).
+There's also a chapter intended to point out differences to
 those familiar with the previous docs written in LaTeX.