Add hint to use alphanumeric passwords

mailcow.conf

@@ -8,6 +8,7 @@ MAILCOW_HOSTNAME=mail.example.org
 # SQL database configuration
 DBNAME=mailcow
 DBUSER=mailcow
+# Please use long, random alphanumeric strings (A-Za-z0-9)
 DBPASS=mysafepasswd
 DBROOT=myothersafepasswd