Added docs about urls.py and testing views for contrib apps

by using a ``tests`` folder in the normal Python way).

For the tests to be found, a ``models.py`` file must exist (it doesn't

have to have anything in it).

have to have anything in it).  If you have URLs that need to be

mapped, you must add them in ``tests/urls.py``.

To run tests for just one contrib app (e.g. ``markup``), use the same

method as above::