Document about tox testing

CONTRIBUTING.rst

@@ -46,6 +46,8 @@ General Guidelines
 - Write tests and make sure they pass (make sure you have a mongod
   running on the default port, then execute ``python setup.py test``
   from the cmd line to run the test suite).
+- Ensure tests pass on every Python and PyMongo versions.
+  You can test on these versions locally by executing ``tox``
 - Add yourself to AUTHORS :)
 
 Documentation

README.rst

@@ -98,6 +98,17 @@ Tests
 To run the test suite, ensure you are running a local instance of MongoDB on
 the standard port, and run: ``python setup.py test``.
 
+To run the test suite on every supported Python version and every supported PyMongo version,
+you can use ``tox``.
+tox and each supported Python version should be installed in your environment:
+
+.. code-block:: shell
+
+    # Install tox
+    $ pip install tox
+    # Run the test suites
+    $ tox
+
 Community
 =========
 - `MongoEngine Users mailing list