Update publisher-github readme to provide required permissions

[steveoh]

Pre-flight checklist

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project uses.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem description

The npm package does not document the required permissions it needs to function leading to errors in deployments

✖ [publisher-github] Running the publish command [FAILED: Resource not accessible by integration]
✖ Publishing distributables [FAILED: Resource not accessible by integration]

Proposed solution

update the readme to include the following permissions. I believe these are the minimum requirements.

permissions:
  id-token: write
  deployments: write
  contents: write

Alternatives considered

none

Additional information

I'm happy to contribute the doc change if you can verify the correct permissions.