Write docs on running the test suite with tox, resolve #399

CONTRIBUTING.rst

@@ -9,3 +9,48 @@ Getting your pull request merged in
 #. Keep it small. The smaller the pull request the more likely I'll pull it in.
 #. Pull requests that fix a current issue get priority for review.
 #. If you're not already in the `CONTRIBUTORS.rst` file, add yourself!
+
+Testing
+-------
+
+Installation
+~~~~~~~~~~~~
+
+Please install `tox`_, which is a generic virtualenv management and test command line tool.
+
+`tox`_ is available for download from `PyPI`_ via `pip`_::
+
+    $ pip install tox
+
+It will automatically create a fresh virtual environment and install our test dependencies,
+such as `pytest-cookies`_ and `flake8`_.
+
+Run the Tests
+~~~~~~~~~~~~~
+
+Tox uses py.test under the hood, hence it supports the same syntax for selecting tests.
+
+For further information please consult the `pytest usage docs`_.
+
+To run all tests using various versions of python in virtualenvs defined in tox.ini, just run tox.::
+
+    $ tox
+
+It is possible to tests with some versions of python, to do this the command
+is::
+
+    $ tox -e py27,py34
+
+Will run py.test with the python2.7, and python3.4 interpreters, for
+example.
+
+To run a particular test with tox for against your current Python version::
+
+    $ tox -e py -- -k test_default_configuration
+
+.. _`pytest usage docs`: https://pytest.org/latest/usage.html#specifying-tests-selecting-tests
+.. _`tox`: https://tox.readthedocs.org/en/latest/
+.. _`pip`: https://pypi.python.org/pypi/pip/
+.. _`pytest-cookies`: https://pypi.python.org/pypi/pytest-cookies/
+.. _`flake8`: https://pypi.python.org/pypi/flake8/
+.. _`PyPI`: https://pypi.python.org/pypi