Add permission documentation for GITHUB_TOKEN

[actuallymentor]

When following the docs verbatim, Github Actions fail with a 403 error since the default setting of the GITHUB_TOKEN is read-only.

Example log for googlability:

RequestError [HttpError]: Resource not accessible by integration
    at /home/runner/work/_actions/FirebaseExtended/action-hosting-deploy/v0/bin/action.min.js:84764:27
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async createCheck (/home/runner/work/_actions/FirebaseExtended/action-hosting-deploy/v0/bin/action.min.js:87337:17)
    at async run (/home/runner/work/_actions/FirebaseExtended/action-hosting-deploy/v0/bin/action.min.js:93186:14) ***
  status: 403,
  response: ***
    url: 'https://api.github.com/repos/actuallymentor/couchcaster.app/check-runs',
    status: 403,
    headers: '...',
    data: ***
      message: 'Resource not accessible by integration',
      documentation_url: 'https://docs.github.com/rest/checks/runs#create-a-check-run',
      status: '403'
    ***
  ***,
  request: ***
    method: 'POST',
    url: '...',
    headers: ***
      accept: 'application/vnd.github.v3+json',
      'user-agent': 'octokit-core.js/3.6.0 Node.js/20.13.1 (linux; x64)',
      authorization: 'token [REDACTED]',
      'content-type': 'application/json; charset=utf-8'
    ***,
    body: '...'

[google-cla]

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[actuallymentor]

Signed the CLA just now. Not sure how to retrigger the check.