Note elevation and Group Policy requirements for local manifest testing

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

Author: Trenly

doc/FirstContribution.md

@@ -25,6 +25,8 @@ Use this checklist before you open your first manifest PR to `microsoft/winget-p
    winget install --manifest <path-to-manifest>
    ```
 
+   > **Note:** `winget settings --enable LocalManifestFiles` requires elevation (run as administrator). On enterprise-managed devices, Group Policy may also block this setting. If you cannot enable local manifests, use Windows Sandbox with `Tools\SandboxTest.ps1` instead (step 11).
+
 11. If possible, test in Windows Sandbox with [`Tools\SandboxTest.ps1`](tools/SandboxTest.md).
 12. If your PR is a routine manifest submission, you do **not** need a separate issue first. Open an issue when you are proposing a repo change, discussing a policy/spec question, or reporting a broader problem.
 13. After you submit the PR, watch for validation labels and review [ValidationFailureGuide.md](ValidationFailureGuide.md) if the bot reports a problem.