Add description about tox in dev_env.rst
@@ -94,6 +94,13 @@ Grab the code::
Running unit tests
------------------
+The preferred way to run the unit tests is using ``tox``. It executes tests in
+isolated environment, by creating separate virtualenv and installing
+dependencies from the ``requirements.txt`` and ``test-requirements.txt`` files,
+so the only package you install is ``tox`` itself::
+
+ sudo pip install tox
+
Run the unit tests by doing::
tox -e py35
