Mention about permalink in the menu

README.md

@@ -56,6 +56,7 @@ It should contain the following header:
     order: 1
     ---
     
+* Warning! In case of pages that should be available in the menu, `permalink` item (described below) **IS MANDATORY**.
 * Leave layout as `default`.
 * The title attribute (here - `index`) is displayed in the menu, thus rename it to your needs.
 * Order attribute (here - `1`) decides about the order in the menu - lower numbers come first.