Explicitly write in the README the required helm version to be installed

[za]

issue #1902

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

[commjoen]

Thank you @za 👍