Add testing section to README

Author: marselester

README.rst

@@ -514,3 +514,21 @@ Please report all issues with this code using the `GitHub issue tracker
 If you are having an issue with a MaxMind service that is not specific to the
 client API, please contact `MaxMind support
 <https://www.maxmind.com/en/support>`_ for assistance.
+
+Testing
+-------
+
+Get the repository and its submodule.
+
+.. code-block:: bash
+
+    $ git clone https://github.com/maxmind/GeoIP2-python.git
+    $ cd ./GeoIP2-python/
+    $ git submodule update --init --recursive
+
+Install and run ``tox``.
+
+.. code-block:: bash
+
+    $ pip install tox
+    $ tox