Mention minimal SMTP configuration in README.md

Looking at https://github.com/nextcloud/docker/blob/d1dbc77e0b5e851105cf8606942e74b7d21fe682/.config/smtp.config.php
it is clear that `SMTP_HOST`, `MAIL_FROM_ADDRESS` and `MAIL_DOMAIN` must be set.

nextcloud/content.md

@@ -169,6 +169,8 @@ To use an external SMTP server, you have to provide the connection details. To c
 -	`MAIL_FROM_ADDRESS` (not set by default): Use this address for the 'from' field in the emails sent by Nextcloud.
 -	`MAIL_DOMAIN` (not set by default): Set a different domain for the emails than the domain where Nextcloud is installed.
 
+At least `SMTP_HOST`, `MAIL_FROM_ADDRESS` and `MAIL_DOMAIN` must be set for the configurations to be applied.
+
 Check the [Nextcloud documentation](https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/email_configuration.html) for other values to configure SMTP.
 
 To use an external S3 compatible object store as primary storage, set the following variables: