Update docs/configuring-playbook-federation.md: add note about setting server delegation

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-federation.md

@@ -2,6 +2,7 @@
 
 By default, your server federates with the whole Matrix network. That is, people on your server can communicate with people on any other Matrix server.
 
+**Note**: in the sample `vars.yml` ([`examples/vars.yml`](../examples/vars.yml)), we recommend to use a short user identifier like `@<username>:example.com` and set up [server delegation](howto-server-delegation.md) / redirection. Without a proper configuration, your server will effectively not be part of the Matrix network. If you find your server is not federated, make sure to [check whether services work](maintenance-checking-services.md) and your server is properly delegated.
 
 ## Federating only with select servers