* Remove unnecessary config options from systemd service so it will work with earlier versions of systemd. Simplify the systemd service instructions and make them more complete.
* Minor systemd README improvements.
* Add back some of the optional systemd 229 stuff but commented out for compat.
* A bunch of updates to the README for linux systemd.
The exemplary unit file for systemd is intentionally redundant at times, for
example dropping privileges which an unprivileged user "www-data" did not have
in the first place: To aid as fallback in case the file gets copied and an
operator setting UID to 0 (which reportedly happened in the past).
Unlike network.target the network-online.target guarantees that the network
devices are online.
If you bind to 0.0.0.0, [::], [::1], and/or 127.0.0.1 only that is enough to
proceed. But in case a particular IP is needed, like ${COREOS_PUBLIC_IPV4},
we require any IP assignments to have completed before Caddy's start. That
is achieved by depending on systemd-networkd-wait-online.service (which is
scheduled before network-online.target, then, automatically).
Add systemd service file for caddy
Add some README with basic setup instructions
Explain how to view the service configuration
Add a note about permissions
Add a comment about run user and group
service->service unit
A systemd service can consist of different units. A unit configuration
file has the `.service` file ending which is a bit confusing, so please
be considerate if I'm confusing `service` and `unit` in the README
Fix typos/reword
Add contact information
