Docs: Clean README and move dev instructions to CONTRIBUTING

[martinschaer]

What is the motivation?

README was outdated.

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

What does this change do?

What is your testing strategy?

n/a

Is this related to any issues?

n/a

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

[github-actions]

🚀 This PR is included in v2.0.0-alpha.1