Documentation - Adding class diagram to README.md

[arthurhenz]

In order to the devs understand better the code structure, class diagram was added to the README.md

WARNING: For each code change, the diagram must be updated as well to avoid its deprecating. This diagram was generated with VS Code plugin "classdiagram-ts"

[evaera]

Hi, thanks for the PR.. Unfortunately I don't think that this is very helpful documentation. Class diagrams are not very commonly used in practice. It's easier to navigate through a code base using your editor.