Document how to change the user

System runs as user purism by default, this is how to change this:

systemctl stop phosh
addduser test1
for group in sudo audio video systemd-journal render netdev feedbackd geoclue; do adduser test1 $group; done

mkdir -p /etc/systemd/system/phosh.service.d
cat <<EOF > /etc/systemd/system/phosh.service.d/override.conf
[Service]
User=test1
EOF

systemctl daemon-reload 
systemctl start phosh

Likely to hacky for the user docs so developer docs might be a better place. Note that if the purism user gains new groups (though unlikely) you need to update this user too.