docs: add AppServiceProvider update instructions to README

[devin-ai-integration]

Summary

Adds a new "Update AppServiceProvider" section to the README explaining that users need to add URL::forceScheme('https') to their AppServiceProvider so Laravel generates HTTPS URLs when running behind the Nginx SSL reverse proxy.

Review & Testing Checklist for Human

• Verify the code snippet style: The snippet uses a fully-qualified class name (\Illuminate\Support\Facades\URL::forceScheme('https')) instead of adding a use import at the top. Confirm this is the preferred style for the README.
• Verify placement in README: The new section is placed between "Install" and "Trust the certificate (optional)". Confirm this ordering makes sense for the reader's workflow.
• Consider snippet scope: The snippet shows the entire AppServiceProvider file. Users with existing customizations in their provider may find a minimal snippet (just the boot() method change) clearer. Decide if this is acceptable.

Notes

• Requested by: @ryoluo
• Link to Devin run

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.