Update docs/configuring-playbook-email.md: add the links to the role and its documentation

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

docs/configuring-playbook-email.md

@@ -20,6 +20,10 @@ By default, emails are sent from `matrix@matrix.example.com`, as specified by th
 
 💡 To improve deliverability, we recommend [relaying email through another SMTP server](#relaying-email-through-another-smtp-server) anyway.
 
+The Ansible role for exim-relay is developed and maintained by [the MASH (mother-of-all-self-hosting) project](https://github.com/mother-of-all-self-hosting/ansible-role-exim-relay). For details about configuring exim-relay, you can check them via:
+- 🌐 [the role's documentation at the MASH project](https://github.com/mother-of-all-self-hosting/ansible-role-exim-relay/blob/main/docs/configuring-exim-relay.md) online
+- 📁 `roles/galaxy/exim_relay/docs/configuring-exim-relay.md` locally, if you have [fetched the Ansible roles](installing.md#update-ansible-roles)
+
 ## Firewall settings
 
 No matter whether you send email directly (the default) or you relay email through another host (see how below), you'll probably need to allow outgoing traffic for TCP ports 25/587 (depending on configuration).