Add info about centrifugo_exporter

[nordicdyno]

add info about https://github.com/nordicdyno/centrifugo_exporter

[FZambia]

Hello, thanks for pull request! This is too specific for a new chapter. I suggest adding link to this list.

For future a good idea can be creating chapter in docs with title Monitoring and description how to monitor Centrifugo - and maybe add this text there - but it requires more general description before mentioning Prometheus exporter.