Update docs/installing.md: add instruction to serve the base domain

The instruction describes the easiest way of configuring Service Discovery (which is required for federation) should help to understand what basically to be done to finalize the installation.

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

docs/installing.md

@@ -100,10 +100,21 @@ A way to invoke these `ansible-playbook` commands with less typing in the future
 
 Now that services are running, you need to **finalize the installation process** (required for federation to work!) by [Configuring Service Discovery via .well-known](configuring-well-known.md).
 
+If you need the base domain for anything else (such as hosting a website), you have to configure it manually, following the procedure described on the linked documentation.
+
+However, if you do not need the base domain (`example.com`) for anything else, the easiest way of configuring it is to [serve the base domain](configuring-playbook-base-domain-serving.md) from the integrated web server. It will enable you to use a Matrix user identifier like `@<username>:example.com` while hosting services on a subdomain like `matrix.example.com`.
+
+To configure Service Discovery in this way, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file:
+
+```yaml
+matrix_static_files_container_labels_base_domain_enabled: true
+```
+
+After configuring the playbook, run the [installation](installing.md) command: `just install-all` or `just setup-all`
 
 ## 4. Things to do next
 
-After you have started the services and **finalized the installation process** (required for federation to work!) by [Configuring Service Discovery via .well-known](configuring-well-known.md), you can:
+After finilizing the installation, you can:
 
 - [check if services work](maintenance-checking-services.md)
 - or [create your first Matrix user account](registering-users.md)