List minimum permissions in README.md

GitHub recommends users limit the permissions of any third-party actions: [^1]

> - **Use credentials that are minimally scoped**
> - Make sure the credentials being used within workflows have the least privileges required, and be mindful that any user with write access to your repository has read access to all secrets configured in your repository.
> - Actions can use the `GITHUB_TOKEN` by accessing it from the `github.token` context. For more information, see "[Contexts](https://docs.github.com/en/actions/learn-github-actions/contexts#github-context)." You should therefore make sure that the `GITHUB_TOKEN` is granted the minimum required permissions. It's good security practice to set the default permission for the `GITHUB_TOKEN` to read access only for repository contents. The permissions can then be increased, as required, for individual jobs within the workflow file. For more information, see "[Automatic token authentication](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token)."

Can you please list the required permissions [^2] for the `devcontainers/ci` in a new section of the `README.md` and in the relevant code examples?

[^1]: [GitHub Docs, security hardening, using secrets](https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-secrets)
[^2]: [GitHub Docs, workflow syntax, permissions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions)