Add a class diagram to the README

[oliverklee]

We'll be able to use Mermaid for this:

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams#creating-mermaid-diagrams
https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/
https://mermaid.js.org/syntax/classDiagram.html