docs(readme/class-diagram): add lacking rule relation

[binhtran432k]

🤔 What's changed?

Add lacking relation between Rule and Tag in Class Diagram

⚡️ What's your motivation?

While developing a Tree-sitter parser for gherkin, I've just realized that the relationship between Rule and Tag is lacking

🏷️ What kind of change is this?

• 📖 Documentation (improvements without changing code)

♻️ Anything particular you want feedback on?

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[kieran-ryan]

Thanks @binhtran432k!