Add some documentation.

docs/workers.md

@@ -303,6 +303,24 @@ stream_writers:
     events: event_persister1
 ```
 
+#### Background tasks
+
+There is also *experimental* support for moving background tasks to a separate
+worker. These are either run periodically or started via replication and which
+tasks are run depend on your Synapse configuration (e.g. if stats is enabled).
+
+To enable this, the worker must have a `worker_name` and be listed in the
+`instance_map` config. For example, to move background tasks to a dedicated
+worker, the shared configuration would include:
+
+```yaml
+instance_map:
+    background_worker:
+        host: localhost
+        port: 8035
+
+run_background_tasks: background_worker
+```
 
 ### `synapse.app.pusher`