Inform users about interactions with unattended-upgrades

[maertsen]

unattended-upgrades is a script (and debian package) that install {stable,security} upgrades unattended. Third party repositories are not included by default, which creates a difference between e.g. a routinator as packaged by Debian vs. routinator as packaged by us. In the former case, users get security fixes automatically, in the latter case they have to apt upgrade manually.

We should make sure there's no implicit expectation that the installation of unattended-upgrades will auto-install updates from our own repo.

We should:

• document how to hook up our repository to unattended-upgrades

We can either:

1. merely warn that there's no integration with unattended-upgrades by default
2. advise to hook-up unattended-upgrades and point to that documentation
3. ship configuration (or a dialog?) to enable unattended-upgrades

See documentation on how to add an origin at https://github.com/mvo5/unattended-upgrades