Added a warning about the {% url %} syntax change

at the point where it bites most beginners.

Refs #18787, #18762, #18756, #18723, #18705, #18689 and several duplicates.

docs/intro/tutorial03.txt

@@ -550,5 +550,16 @@ with the :ttag:`url` template tag:
 
     <li><a href="{% url 'polls.views.detail' poll.id %}">{{ poll.question }}</a></li>
 
+.. note::
+
+    If ``{% url 'polls.views.detail' poll.id %}`` (with quotes) doesn't work,
+    but ``{% url polls.views.detail poll.id %}`` (without quotes) does, that
+    means you're using a version of Django ≤ 1.4. In this case, add the
+    following declaration at the top of your template:
+
+    .. code-block:: html+django
+
+        {% load url from future %}
+
 When you're comfortable with writing views, read :doc:`part 4 of this tutorial
 </intro/tutorial04>` to learn about simple form processing and generic views.