use shortcut with mdwn disabled

doc/plugins/shortcut/discussion.mdwn

@@ -0,0 +1,16 @@
+The plugin somehow depends on [[mdwn]].  If you have
+disabled [[mdwn]], to get [[shortcut]] work, you need
+commit in a shortcuts.ext (ext is rcs|creole|html|txt|etc).
+
+Then edit the following lines in[[shortcut]] plugin source:
+
+    my $srcfile=srcfile("shortcuts.mdwn", 1);
+
+and
+
+    error(gettext("shortcut plugin will not work without a shortcuts.mdwn"));
+
+
+(It is better to edit a local copy of shortcut.pm in, say, ~/.ikiwiki/IkiWiki/Plugin
+and add ~/.ikiwiki to libdir in your ikiwiki.setup then edit the shortcut.pm installed
+in system directly.)