[Low Priority] View documentation (ansible#374)

Fixes ansible#371

Author: john-westcott-iv

docs/lib/views.md

@@ -48,3 +48,32 @@ class DefaultAPIView(AnsibleBaseView):
     all my good stuff
     ...
 ```
+
+
+## Adding extra related fields
+
+There may be cases where the associative router does not provide a way to add specific related views to your view. As a way to help combat this the `AnsibleBaseView` class has a method defined called `extra_related_fields` which returns an empty dict (`{}`) by default. The entries expected in this dictionary are in the format:
+```
+{
+    "string": "url",
+}
+```
+Where string is the name of the field under the related section in the API and URL is the URL to that object.
+
+For example, if `extra_related_fields` returned:
+```
+{
+    'authenticators': reverse('user-authenticators-list', kwargs={'pk': obj.pk}),
+}
+```
+
+We would expect to see the related fields for an object to be something along the lines of:
+```
+    "related": {
+        "authenticators": "/api/gateway/v1/users/1/authenticators/"
+    }
+```
+
+The URL for that entry, in our example, will be generated from the reverse lookup user-authenticators-list url and should end up as something like `/api/users/:id/authenticators/` where :id comes from the `obj.pk`. 
+
+This can be particularly useful if a feature in DAB wants to extend a view that is created by a service. In this case you can create a MixIn like `ansible_base.oauth2_provider.views.user_mixin.DABOAuth2UserViewsetMixin` which can set `extra_related_fields` and then have the view in the service extend the Mixin.