wxGUI menudata.README: add links to docs

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@66693 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/xml/menudata.README

@@ -1,7 +1,8 @@
 MENU
 - the entries in menudata.xml and module_tree_menudata.xml are autogenerated 
   via tools/build_modules_xml.py (see ../Makefile)
-
-TOOLBOXES
-- Find more info about how to add items to the menu:
+- Find more info at
+  https://grass.osgeo.org/grass71/manuals/wxGUI.toolboxes.html
+  (../docs/wxgui_sphinx/src/)
+- additionally
   https://trac.osgeo.org/grass/wiki/wxGUIDevelopment/Toolboxes