Update Documentation for Voicemail redirects

[pwfraley]

We just noticed, that when someone has a redirect to voicemail after x seconds, then sipgate.io will register it as a new call with a new call id. In the newcall event, there is now a callId and a origCallId.

Unfortunately there is no mention about this in the documentation. This caused an error in our system, because every unanswered call showed up twice in our system.

[danielberlin]

It's mentioned here: 8c58cd8

[BlackMac]

What can we do to make the documentation more obvious?

[pwfraley]

Maybe it would be more obvious if the tables for the events in the README.md would list all params possible?
Also I find no mention in the README.md of the originalCallId, it would be nice to also have it mentioned there (https://github.com/sipgate/sipgate.io/blob/master/README.md)

@danielberlin origCallId is not mentioned in that commit.