add warning if not using password_file

nixos/modules/services/mail/goeland.nix

···
       isSystemUser = true;
     };
     users.groups.goeland = { };
+
+    warnings =
+      if hasAttr "password" cfg.settings.email
+      then [
+        ''
+          It is not recommended to set the "services.goeland.settings.email.password"
+          option as it will be in cleartext in the Nix store.
+          Please use "services.goeland.settings.email.password_file" instead.
+        ''
+      ]
+      else [ ];
   };
 
   meta.maintainers = with maintainers; [ sweenu ];