docs(README): SMTP/MAIL values always override web UI (#2243)

Fixes #1312
Signed-off-by: Josh <josh.t.richards@gmail.com>

README.md

@@ -163,7 +163,7 @@ If you want to use Redis you have to create a separate [Redis](https://hub.docke
 
 The use of Redis is recommended to prevent file locking problems. See the examples for further instructions.
 
-To use an external SMTP server, you have to provide the connection details. To configure Nextcloud to use SMTP add:
+To use an external SMTP server, you have to provide the connection details. Note that if you configure these values via Docker, you should **not** use the Nexcloud Web UI to configure external SMTP server parameters. Conversely, if you prefer to use the Web UI, do **not** set these variables here (because these variables will override whatever you attempt to set in the Web UI for these parameters). To configure Nextcloud to use SMTP add:
 
 - `SMTP_HOST` (not set by default): The hostname of the SMTP server.
 - `SMTP_SECURE` (empty by default): Set to `ssl` to use SSL, or `tls` to use STARTTLS.