@vincent @fschuttkowski First the obligatory disclaimer, IANAL. 😅 Now, regarding:
You don't need a GDPR notice if you're not a business.
It depends. There are exemptions for natural persons when it comes to processing personal data as part of a purely private activity, like adding contact information to your address book.
But that exemption may not apply if you decide to publish your address book to a large audience on, for example, your blog.
Source: The purposes and scope of GDPR.
So, to be on the safe side, you should talk to a lawyer. Or, you know, just not post personal data like names, addresses, or photographs of identifiable (and alive) EU citizens on your blog. 😊 If there's no personal data, GDPR does not apply.
Another thing that may impact you, as a German, is the Impressumspflicht. Here's an overview on the IndieWeb wiki.