Mention minimal SMTP configuration in README.md (#2144)

Looking at d1dbc77e0b/.config/smtp.config.php
it is clear that `SMTP_HOST`, `MAIL_FROM_ADDRESS` and `MAIL_DOMAIN` must be set.

Signed-off-by: Raphael Nestler <raphael.nestler@gmail.com>

README.md

@@ -174,6 +174,8 @@ To use an external SMTP server, you have to provide the connection details. To c
 - `MAIL_FROM_ADDRESS` (not set by default): Set the local-part 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: