Add note about cookie requirement

notebook/templates/login.html

@@ -63,6 +63,9 @@ http://localhost:8888/?token=c8de56fa... :: /Users/you/notebooks
       <p>
         Or you can paste just the token value into the password field on this page.
       </p>
+      <p>
+        Cookies are required for authenticated access to notebooks.
+      </p>
     </div>
     {% endblock token_message %}
     {% endif %}