Documented TLS Authentication with node https.Agent

[andreacioni]

Hi everyone,

I open this PR because I found a bit hard to configure TLS authentication against a Schema Registry that accept just this kind of authentication method. I hope that with this integration in the documentation people won't have the same problem (here an old issue raised on this topic #61 ).

Please tell me if you want me to add tests also.

[silverwind]

This is good docs and I was looking for those. I do have to comment that I don't really like how this is done via https.Agent which does not document these options at all, thought I guess this is more of a Node.js docs issue.