docs: add basic feature, usage docs

docs/narr/usage.rst

@@ -0,0 +1,41 @@
+
+Usage
+=====
+
+You can check the feature status with
+:meth:`~wutta_continuum.app.WuttaContinuumAppProvider.continuum_is_enabled()`::
+
+   app = config.get_app()
+
+   if not app.continuum_is_enabled():
+       print("Oh no!  Continuum is not enabled.")
+
+The rest of this will assume the feature is enabled.
+
+
+Built-In Models
+---------------
+
+The following built-in models are versioned.  So, when records are
+added / modified / removed via the ORM, new version records are
+automatically created for each of these:
+
+* :class:`~wuttjamaican:wuttjamaican.db.model.auth.Permission`
+* :class:`~wuttjamaican:wuttjamaican.db.model.base.Person`
+* :class:`~wuttjamaican:wuttjamaican.db.model.auth.Role`
+* :class:`~wuttjamaican:wuttjamaican.db.model.auth.User`
+* :class:`~wuttjamaican:wuttjamaican.db.model.auth.UserRole`
+
+
+Object Versions
+---------------
+
+A versioned model works normally but also has a ``versions``
+attribute, which reflects the list of version records::
+
+    user = session.query(model.User).first()
+
+    for version in user.versions:
+        print(version)
+
+See also :doc:`sqlalchemy-continuum:version_objects`.