Update README.md with nr of challenges

[commjoen]

What kind of changes does this PR include?

• Fixes or refactors
• A new challenge
• Additional documentation
• Something else

Description

Relations

References

Checklist:

• All the contributions made are solely the work of me and my co-authors
• I tested the changes in this PR (if applicable)
• I added unit tests to ensure my change works (when change in Java or on front-end code)
• I added UI tests to ensure my UI changes work (when change in the overall UI, not needed if just adding a challenge)
• The PR passes pre-commit hooks and automated tests