Add class diagram for the first example in the README

[destogl]

Related to #37

[peakyquest]

👋 Hi, I’d like to work on this issue.
Just to confirm — is the main task here to create a class diagram for the first example and add it into the README, or is there anything else you’d like included (e.g., additional explanation, updating docs)?

Thanks!

[christophfroehlich]

a simple class diagram should be enough, and adding a new section with a brief explanation of the diagram in the docs.
Please also add the source of the diagram (we used drawio for other sketches).

We already have a not-so-simple and not-so-updated UML here and here (png) and here (source).
As a second step, you could have a look what is outdated there and update it later, too 👍