Error "Missing GitHub OIDC request environment variables" is unhelpful

[ascott18]

If you try to set this action up by looking at the examples in the "Usage" part of the README, you then get a vague error "Missing GitHub OIDC request environment variables".

I'd love to see:

1. This error message improved, to provide suggested steps to remedy the problem (configure permissions for the workflow)
2. The README updated to include a full example at the top, rather than an almost-full example at the top that excludes a critical piece of configuration that's only included at the bottom of the file.