Enrich README.md with an example of how to connect over to NATS servers that require authentication

[dsidirop]

Considering that nats-top doesn't support nats "contexts" (feel free to correct me if I'm wrong) it would be helpful to add a few examples on how to use '-key', '-cert' and '-cacert' parameters for those that have various kinds of authentication-schemes enabled on a nats-server - feel free to correct me again if I'm wrong.

Thanks in advance.

PS: It would also be nice to add a warning in the README.md file over the fact that the monitoring ports (8222) of NGS nats-clusters are by default out of reach for any outsider. So nats-top won't work there no matter how much one tries. Personally I always forget this very simple fact.

@wallyqs

[sciolto]

I vote for this. It would be great to have examples on how to run nats-top with client certificate, or password-based authentication.