Added tests/QUICKSTART.rst

Every time I want to run the test suite, I have to google until I find
the docs about how to do it. This commit adds a QUICKSTART.rst file
with the necessary commands in it. For all other documentation, it
simply points to the upstream docs.
To avoid content duplication, you could just add the following content to the document:

```rst
To run the test suite::

    $ PYTHONPATH=..:$PYTHONPATH ./runtests.py

For more information about the test suite, see
https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/unit-tests/.
```