docs: clarify backend selection, Fly.io costs, and Object Storage request scaling

docs/README.md

@@ -12,7 +12,7 @@ A document's original date and reviewed-version fields describe its baseline. Re
 | Choose a CouchDB setup | [Own server](setup_own_server.md), [Docker stack](../docker/README.md), [Fly.io](setup_flyio.md), or [IBM Cloudant (managed)](setup_cloudant.md) |
 | Configure CouchDB clients | [Quick setup](quick_setup.md) |
 | Find hosting services proposed by the community | [Third-party hosting listings](../README.md#hosting-services-proposed-by-the-community) |
-| Set up S3-compatible storage | [Object Storage Setup](setup_object_storage.md) |
+| Set up S3-compatible storage | [Object Storage Setup](setup_object_storage.md) and [transfers and service costs](setup_object_storage_costs.md) |
 | Set up peer-to-peer synchronisation | [P2P communication model](p2p.md), then [Peer-to-Peer Setup](setup_p2p.md) |
 | Diagnose P2P connectivity | [Browser-assisted check](../src/apps/webpeer/README.md#p2p-connection-check), [network tips](tips/p2p-sync-tips.md), and [self-hosted TURN](../docker/coturn/README.md) |
 | Understand a setting or command | [Settings reference](settings.md) |