Add BC note for #5886 to 3.8 Release Announcement

Author: carltongibson

docs/topics/3.8-announcement.md

@@ -33,6 +33,28 @@ If you use REST framework commercially and would like to see this work continue,
 
 *We'd like to say thanks in particular our premium backers, [Rover](http://jobs.rover.com/), [Sentry](https://getsentry.com/welcome/), [Stream](https://getstream.io/?utm_source=drf&utm_medium=banner&utm_campaign=drf), [Machinalis](https://hello.machinalis.co.uk/), and [Rollbar](https://rollbar.com).*
 
+
+---
+
+## Breaking Changes
+
+### Altered behaviour of `read_only` plus `default` on Field.
+
+[#5886][gh5886]
+
+`read_only` fields will now **always** be excluded from writable fields.
+
+Previously `read_only` fields with a `default` value would use the `default` for create and update operations.
+
+In order to maintain the old behaviour you may need to pass the value of `read_only` fields when calling `save()` in
+the view:
+
+    def perform_create(self, serializer):
+        serializer.save(owner=self.request.user)
+
+Alternatively you may override `save()` or `create()` or `update()` on the serialiser as appropriate.
+
+
 ---
 
 ## Boolean Composable Permissions
@@ -72,3 +94,4 @@ TODO
 TODO
 
 [funding]: funding.md
+[gh5886]: https://github.com/encode/django-rest-framework/issues/5886