Fixed #25447 -- Emphasized the need to restart dev server when adding template tags.

The app should contain a ``templatetags`` directory, at the same level as

``models.py``, ``views.py``, etc. If this doesn't already exist, create it -

don't forget the ``__init__.py`` file to ensure the directory is treated as a

Python package. After adding this module, you will need to restart your server

before you can use the tags or filters in templates.

Python package.

.. admonition:: Development server won't automatically restart

    After adding the ``templatetags``  module, you will need to restart your

    server before you can use the tags or filters in templates.

Your custom tags and filters will live in a module inside the ``templatetags``

directory. The name of the module file is the name you'll use to load the tags