Add useful comment about systemd support.
Add useful comment about systemd support.

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

bboozzoo/meta-mender,GregorioDiStefano/meta-mender,GregorioDiStefano/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,GregorioDiStefano/meta-mender,bboozzoo/meta-mender