Add section for handling updates via a service

README.md

@@ -424,6 +424,34 @@ As you can see, this service calls 2 other services - `profile_create` and `conf
 
 In this example, everything related to the user creation is in one place and can be traced.
 
+### Handling updates
+
+As for updating, we have a generic update service that we use inside of the actual update services. Here's what a sample `user_update` service would look like:
+
+```python
+def user_update(*, user: User, data) -> User:
+    non_side_effect_fields = ['first_name', 'last_name']
+
+    user, has_updated = model_update(
+        instance=user,
+        fields=non_side_effect_fields,
+        data=data
+    )
+
+    # Side-effect fields update here (e.g. username is generated based on first & last name)
+
+    # ... some additional tasks with the user ...
+
+    return user
+```
+
+* We're calling the generic `model_update` service for the fields that have no side-effects related to them (meaning that they're just set to the value that we provide).
+* This pattern allows us to extract the repetitive field setting in a generic service and perform only the specific tasks inside of the update service (side-effects).
+
+The generic `model_update` implementation is in the [`Cookbook` section](#generic-update-service).
+
+The full implementation of this approach can be seen [in our example project](https://github.com/HackSoftware/Django-Styleguide-Example/blob/master/styleguide_example/users/services.py).
+
 ### Naming convention
 
 Naming convention depends on your taste. It pays off to have something consistent throughout a project.