- README: add 'What you'll need', overview, and a 'Prepare the Pi and
get the code' step (git clone) so a non-expert can set up from a blank
Pi; flag the non-'pi' default username gotcha.
- Add config.example.yaml and install-ola.sh; update install.sh and the
systemd unit for the env-file/OLA-from-source workflow.
- Remove the local Home Assistant/Mosquitto test files from the repo and
ignore config.yaml (holds the MQTT password).
