Point to upgrade notes

Author: David Robertson

CHANGES.md

@@ -3,6 +3,8 @@ Synapse 1.43.0 (2021-09-21)
 
 This release drops support for the deprecated, unstable API for [MSC2858](https://github.com/matrix-org/matrix-doc/blob/master/proposals/2858-Multiple-SSO-Identity-Providers.md#unstable-prefix), as well as the undocumented `experimental.msc2858_enabled` config option. Client authors should update their clients to use the stable API, available since Synapse 1.30.
 
+The documentation has been updated with configuration for routing `/spaces`, `/heirarchy` and `/summary` to workers. See [the upgrade notes](https://github.com/matrix-org/synapse/blob/release-v1.43/docs/upgrade.md#upgrading-to-v1430) for more details.
+
 No significant changes since 1.43.0rc2.
 
 Synapse 1.43.0rc2 (2021-09-17)