Added docs for running tox test suite.

docs/run_tests.rst

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+Running ``django-dfk`` test suite
+=================================
+
+``django-dfk`` has been configured to work with the ``tox`` package to ensure cross python / django compatibility.
+The ``tox.ini`` file in the root of the source defines which versions of Python ``django-dfk`` should be tested against along
+with all the applicable versions of django that we aim to support.
+
+Running tests with tox
+======================
+
+To run the test suite with tox ``django-dfk`` needs to be installed as an editable egg eg.::
+
+    pip install -e git+ssh://git@github.com/danfairs/django-dfk.git#egg=django-dfk
+
+or be a clone of the checkout with the ``PYTHONPATH`` and ``DJANGO_SETTINGS_MODULE`` environment variables handled accordingly.
+
+Next you need to install tox into your virtualenv::
+
+    pip install tox
+
+Finally you need to make sure you have system installs of all the versions of Python we wish to test against (currently Python 2.6, 2.7, 3.2 and 3.3)
+
+Installing Python versions
+==========================
+
+If you are working on Ubuntu, you can use the Deadsnake PPA repositories to install all of these versions of Python.
+Here is the steps that I took on Ubuntu Desktop 12.04 to get all the python versions setup::
+
+    sudo apt-get install python-software-properties
+    sudo add-apt-repository ppa:fkrull/deadsnakes
+    sudo apt-get update
+    sudo apt-get install python3.3
+    sudo apt-get install python3.2
+    sudo apt-get install python2.6
+
+Please note that the above commands install the ``minimal`` install of each version of Python. Python 3.3 and Python 2.6 also have ``complete`` versions e.g ``sudo apt-get install python2.6-complete``. These versions also install the ``dev`` headers, ``dbg`` libraries etc. It's up to you what you want to install. I tend to air on the side of caution and installed the ``complete`` versions for the sake of being thorough. There is no ``complete`` version for Python 3.2 in Deadsnakes PPA repositories, so if you need the extra libs then you will need to install them manually.
+
+Once all your versions of Python are installed and available, you can run the test suite with tox.
+
+1. Activate the virtualenv that you installed ``django-dfk`` and ``tox`` into.
+2. Run the test suite with tox using the below command
+
+::
+
+    tox
+
+The first build through will be quite slow whilst it builds each of the ``virtualenv``s agaisnt each version of Python along with the specified version of django and runs the tests. Subsequent runs should be much quicker as ``tox`` leaves the ``virtualenv``'s in place by default in a folder named ``.tox``
+
+Run tests quickly
+=================
+
+You don't have to use ``tox`` to run the test suite. If you are making changes to ``django-dfk`` and just want a quick run through on the current version of Python and install Django, then you can run tests as normal using django's inbuilt management command::
+
+    django-admin.py test dfk --settings=dfk.test_settings