Add instructions to add plugin as a local override in example app

[vashworth]

What information needs to be added?

Flutter attempts to automatically add the plugin as a local package in iOS Xcode apps: flutter/flutter#179512

However, this migration may not work in all cases. We should also add instructions on how to do this manually

Where should this new content appear?

https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors

I would like to fix this problem.

• I will try and fix this problem on docs.flutter.dev.

[sfshaza2]

Thanks, @vashworth!