Add note that add-to-app on iOS will require CocoaPods 1.10

https://github.com/flutter/flutter/pull/71495 will add a requirement on CocoaPods 1.10 for add-to-app only due to how add-to-app is using XCFrameworks.  See details about the bug on older CocoaPod versions at https://github.com/flutter/flutter/pull/71531.

https://flutter.dev/docs/development/add-to-app/ios/project-setup#system-requirements