Add documentation about off-by-default options

Add documentation on off-by-default options and how to enable them.

Change-Id: I34fc44f92ac7e8e800b1d8d07914af62cb2679ae

HACKING.rst

@@ -32,7 +32,7 @@ General
   it easier to find out who the author of the comment was.
 - [H105] Don't use author tags. We use version control instead.
 - [H106] Don't put vim configuration in source files (off by default).
-- [H904] Delay string interpolations at logging calls.
+- [H904] Delay string interpolations at logging calls (off by default).
 - Do not shadow a built-in or reserved word. Shadowing built -in or reserved
   words makes the code harder to understand. Example::
 
@@ -304,7 +304,8 @@ exception possible should be used.
       with self.assertRaises(exception.InstanceNotFound):
           db.instance_get_by_uuid(elevated, instance_uuid)
 
-- [H203] Unit test assertions tend to give better messages for more specific
+- [H203] Use assertIs(Not)None to check for None (off by default)
+  Unit test assertions tend to give better messages for more specific
   assertions. As a result, ``assertIsNone(...)`` is preferred over
   ``assertEqual(None, ...)`` and ``assertIs(None, ...)``, and
   ``assertIsNotNone(...)`` is preferred over ``assertNotEqual(None, ...)``

README.rst

@@ -88,6 +88,23 @@ Requirements
   Importing random modules, has caused all kinds of trouble for us in the past.
 
 
+Enabling off-by-default checks
+==============================
+
+Some of the available checks are disabled by default. These checks are:
+
+- [H106] Don't put vim configuration in source files.
+- [H203] Use assertIs(Not)None to check for None.
+- [H904] Delay string interpolations at logging calls.
+
+To enable these checks, edit the ``flake8`` section of the ``tox.ini`` file.
+For example to enable H106 and H203:
+
+.. code-block:: ini
+
+  [flake8]
+  enable-extensions = H106,H203
+
 Local Checks
 ============