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