Add step to warn about unnecessary write permissions

Probes the github-token for write access to actions, checks, contents,
deployments, issues, packages, pages, pull-requests, security-events,
and statuses. Emits a visible warning if any write scopes are detected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -9,7 +9,7 @@ steps:
   - uses: actions/setup-copilot@v0
     with:
       version: "latest" # optional, defaults to "latest"
-      github-token: ${{ secrets.COPILOT_TOKEN }} # optional, defaults to github.token
+      github-token: ${{ secrets.GITHUB_TOKEN }} # optional, defaults to github.token
   - run: copilot --version
 ```