[Documentation] Mention github_token required permissions

[Nargonath]

It would be great to know which of the whole set of permissions this actions requires with which level of access: https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#permissions. That way we can restrict the token permissions to the bare minimum on user side.

[danieleades]

it would also be useful to know the required permissions to help debug why clippy-check is failing to push comments in some repos (even when the branch is not from a forked repo)

see #2 (comment)