Doc: How to add an email invite to an existing calendar

[niccokunzmann]

In the documentation, we should describe this usage of mergecal.
By describing it, we land ourselves in the position of people actually using it and merging calendars from different implementations. That will yield good feedback.

• document how to accept an email invite on the command line with mergecal
• have a search on stackoverflow if we can answer with mergecal, answer and link the docs