Added example how to add a slave domain

[jeroenvermeulen]

Description

Added an example how to add a slave domain.

Motivation and context

It is not hard but it took me quite a while to find out exactly how to do it.

How has this been tested?

Tested on a live PowerDNS 4.7.3

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[jeroenvermeulen]

Can't figure out why exactly the CI went wrong.
You have some serious automation going on there.

[trizz]

Thanks for the example! No worries about the CI, I'll fix that later if it still fails after merging. Your previous PR is released this morning. This example will be released in a future version.

[jeroenvermeulen]

@trizz Wow, really motivating how fast your guys are responding to PRs!
This is a very well maintained project, Dutch developers like to make quality in every aspect! 🇳🇱💪🏻