Docs for Prometheus service discovery using Consul

[Sarke]

The README.md currently mentions how to do this using DNS, but could this process not be improved on by using Consul instead?

As already mentioned in the readme, DockerSwarm.rocks recommends Traefik and HTTPS. This install uses Consul as a key-value store for the SSL certificates, which means many/most of the Docker Swarm installs following that recipe will have Consul installed as well.

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Relevant Prometheus docs:
https://prometheus.io/docs/prometheus/latest/configuration/configuration/#consul_sd_config