Add systemd unit file and some usage instructions

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

dist/init/linux-systemd/README.md

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+# systemd unit for caddy
+
+Please do not hesitate to ask [me](mailto:klingt.net+caddy@gmail.com) if you've any questions.
+
+## Quickstart
+
+- install the unit configuration file: `cp caddy@.service /etc/systemd/system`
+- reload the systemd daemon: `systemctl deamon-reload`
+- make sure to [configure](#configuration) the service unit before starting caddy
+- start caddy: `systemctl start caddy@someuser`
+- enable the service (automatically start on boot): `systemctl enable caddy@someuser`
+- the `.caddy` folder will be created inside the users home directory that runs caddy, i.e. `/home/someuser/.caddy` for `systemctl start caddy@someuser`
+
+## Configuration
+
+- do not edit the systemd unit directly, use systemd's builtin tools:
+    - `systemctl edit caddy@` to make user local modifications to the service unit
+    - `systemctl edit --full caddy@` to make system-wide modifications
+- in most cases it's enough to adapt the `ExecStart` directive:
+    - `systemctl edit caddy@`
+    - systemd needs absolute paths, therefore make sure that the path to caddy is correct
+    - example:
+
+```ini
+[Service]
+; reset the original setting
+ExecStart=
+ExecStart=/usr/bin/caddy -conf="/etc/caddy/myCaddy.conf" -agree -email="my@mail.address"
+```
+
+- to view your configuration use `systemctl cat caddy@`
+- double check the permissions of your web root path to make sure that caddy can access it as its run user and group
+
+## Tips
+
+- use `log stdout` and `errors stderr` in your Caddyfile to make use of `journalctl`
+- `journalctl` is systemd's log query tool
+- lets say you want all the log entries for caddy since the last boot beginning from the last entry: `journalctl --reverse --boot --unit caddy@someuser`
+- maybe you want to follow caddys log output: `journalctl -fu caddy@someuser`
+- to send a signal to a service units main PID, e.g. let caddy reload its config: `systemctl kill --signal=USR1 caddy@someuser`

dist/init/linux-systemd/caddy@.service

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+; see `man systemd.unit` for configuration details
+; the man section also explains *specifiers* `%x`
+
+[Unit]
+Description=Caddy HTTP/2 web server %I
+Documentation=https://caddyserver.com/docs
+After=network.target
+
+[Service]
+; run user and group for caddy
+User=%i
+Group=http
+ExecStart=/usr/bin/caddy -agree=true -conf=/etc/caddy/Caddyfile
+Restart=on-failure
+; create a private temp folder that is not shared with other processes
+PrivateTmp=true
+; limit the number of file descriptors, see `man systemd.exec` for more limit settings
+LimitNOFILE=8192
+
+[Install]
+WantedBy=multi-user.target