Adds comment w/ reference to upstream advice, for maintenance

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

Author: DougReeder

charts/docker-mailserver/values.yaml

@@ -578,6 +578,9 @@ configMaps:
       #!/bin/bash
 
       {{- if .Values.proxyProtocol.enabled }}
+      # NOTE: Keep in sync with upstream advice:
+      # https://github.com/docker-mailserver/docker-mailserver/blob/v15.0.0/docs/content/examples/tutorials/mailserver-behind-proxy.md?plain=1#L238-L268
+
       # Duplicate the config for the submission(s) service ports (587 / 465) with adjustments for the PROXY ports (10587 / 10465) and `syslog_name` setting:
       postconf -Mf submission/inet | sed -e s/^submission/10587/ -e 's/submission/submission-proxyprotocol/' >> /etc/postfix/master.cf
       postconf -Mf submissions/inet | sed -e s/^submissions/10465/ -e 's/submissions/submissions-proxyprotocol/' >> /etc/postfix/master.cf