Added parallel test running to "Speeding up the tests" docs.

Speeding up the tests

---------------------

In recent versions of Django, the default password hasher is rather slow by

design. If during your tests you are authenticating many users, you may want

to use a custom settings file and set the :setting:`PASSWORD_HASHERS` setting

to a faster hashing algorithm::

Running tests in parallel

~~~~~~~~~~~~~~~~~~~~~~~~~

As long as your tests are properly isolated, you can run them in parallel to

gain a speed up on multi-core hardware. See :option:`test --parallel`.

Password hashing

~~~~~~~~~~~~~~~~

The default password hasher is rather slow by design. If you're authenticating

many users in your tests, you may want to use a custom settings file and set

the :setting:`PASSWORD_HASHERS` setting to a faster hashing algorithm::

    PASSWORD_HASHERS = [

        'django.contrib.auth.hashers.MD5PasswordHasher',