Revisit traefik labels in readme

[aine-etke]

In the #279 (comment) it was mentioned that the traefik labels in README don't work. It looks like in Traefik v3 labels have been changed, so that section should be revisited and updated.

Reference with working labels: https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/roles/custom/matrix-synapse-admin/templates/labels.j2