Add a comment for the listFields option (#821)

* Add doc for the listFields option

Currently the doc add an empty `listFields` option, this cause a blank List view without fields.

* Typo

Author: comxd

admin/customizing.md

@@ -24,7 +24,9 @@ const greetings = {
       field: greetingsNameField,
     }
   ],
-  listFields: [],
+  listFields: [
+    // list here all fields you want to be displayed in the List view
+  ],
 };
 
 const books = {