Document that `actions: read/write` permissions are required to use gh-actions-cache in gh actions CI

[corneliusroemer]

I tried to use gh-actions-cache in a Github action.

It failed with:

Error: Resource not accessible by integration.

It's not immediately clear which permission I needed to give to the action so it works.

After consulting the docs at https://docs.github.com/en/rest/authentication/permissions-required-for-github-apps?apiVersion=2022-11-28#repository-permissions-for-actions I figured out, I needed to give:

permissions:
  actions: write

but this wasn't documented anywhere explicitly.

Would be great to add this to the README.