Add (beginning of) 3.9 release announcement.

@tomchristie: Input on OpenAPI and What’s Next very welcome! :)

docs/community/3.9-announcement.md

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+<style>
+.promo li a {
+    float: left;
+    width: 130px;
+    height: 20px;
+    text-align: center;
+    margin: 10px 30px;
+    padding: 150px 0 0 0;
+    background-position: 0 50%;
+    background-size: 130px auto;
+    background-repeat: no-repeat;
+    font-size: 120%;
+    color: black;
+}
+.promo li {
+    list-style: none;
+}
+</style>
+
+# Django REST framework 3.9
+
+The 3.9 release gives access to _extra actions_ in the Browsable API, introduces composable permissions and built-in [OpenAPI][openapi] schema support.
+
+---
+
+## Funding
+
+If you use REST framework commercially and would like to see this work continue, we strongly encourage you to invest in its continued development by
+**[signing up for a paid&nbsp;plan][funding]**.
+
+*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).*
+
+---
+
+TODO: HEADLINE FEATURES. 1, 2, 3.
+
+---
+
+## Breaking Changes
+
+TODO... (NONE???)
+
+---
+
+## Deprecations
+
+### Router argument/method renamed to use `basename` for consistency.
+
+* The `Router.register` `base_name` argument has been renamed in favor of `basename`.
+* The `Router.get_default_base_name` method has been renamed in favor of `Router.get_default_basename`. [#5990][gh5990]
+
+See [#5990][gh5990].
+
+[gh5990]: https://github.com/encode/django-rest-framework/pull/5990
+
+`base_name` and `get_default_base_name()` are pending deprecation. They will be deprecated in 3.10 and removed entirely in 3.11.
+
+### `action` decorator replaces `list_route` and `detail_route`
+
+Both `list_route` and `detail_route` are now deprecated in favour of the single `action` decorator.
+They will be removed entirely in 3.10.
+
+The `action` decorator takes a boolean `detail` argument.
+
+* Replace `detail_route` uses with `@action(detail=True)`.
+* Replace `list_route` uses with `@action(detail=False)`.
+
+### `exclude_from_schema`
+
+Both `APIView.exclude_from_schema` and the `exclude_from_schema` argument to the `@api_view` have now been removed.
+
+For `APIView` you should instead set a `schema = None` attribute on the view class.
+
+For function based views the `@schema` decorator can be used to exclude the view from the schema, by using `@schema(None)`.
+
+---
+
+## Minor fixes and improvements
+
+There are a large number of minor fixes and improvements in this release. See the [release notes](release-notes.md) page
+for a complete listing.
+
+
+## What's next
+
+
+TODO...
+
+
+[funding]: funding.md
+[gh5886]: https://github.com/encode/django-rest-framework/issues/5886
+[gh5705]: https://github.com/encode/django-rest-framework/issues/5705
+[openapi]: https://www.openapis.org/