Add documentation for override_config decorator

docs/index.rst

@@ -133,6 +133,7 @@ More documentation
    :maxdepth: 2
 
    backends
+   testing
    changes
 
 Indices and tables

docs/testing.rst

@@ -0,0 +1,41 @@
+Testing
+=======
+
+Testing how your app behaves with different config values is achieved with the
+:class:`override_config` class. This intentionally mirrors the use of Django's
+:class:`~django.test.override_setting`.
+
+.. py:class:: override_config(**kwargs)
+
+    Replace key-value pairs in config.
+
+
+Usage
+~~~~~
+
+It can be used as a decorator at the :class:`~django.test.TestCase` level, the
+method level and also as a
+`context manager <https://www.python.org/dev/peps/pep-0343/>`_.
+
+.. code-block:: python
+
+    from constance import config
+    from constance.test import override_config
+
+    from django.test import TestCase
+
+
+    @override_config(YOUR_NAME="Arthur of Camelot")
+    class ExampleTestCase(TestCase):
+
+        def test_what_is_your_name(self):
+            self.assertEqual(config.YOUR_NAME, "Arthur of Camelot")
+
+        @override_config(YOUR_QUEST="To find the Holy Grail")
+        def test_what_is_your_quest(self):
+            self.assertEqual(config.YOUR_QUEST, "To find the Holy Grail")
+
+        def test_what_is_your_favourite_color(self):
+            with override_config(YOUR_FAVOURITE_COLOR="Blue?"):
+                self.assertEqual(config.YOUR_FAVOURITE_COLOR, "Blue?")
+