Add comment explaining KDE dependencies situation

mkosi.conf.d/45-packages-kde-dependencies.conf

@@ -1,6 +1,11 @@
 # SPDX-License-Identifier: CC0-1.0
 # SPDX-FileCopyrightText: Ian Monroe <imonroe@kde.org
 
+# Only add packages here if they have a valid reason to remain optional
+# dependencies of KDE software. If there is no valid reason, instead add
+# them to
+# https://invent.kde.org/sysadmin/repo-metadata/-/blob/master/distro-dependencies/arch.yaml
+
 [Content]
 
 Packages=accountsservice # Users KCM