Update documentation for using bdk-flutter on App Store builds

[MrUnfunny]

It works fine on normal debug and release builds but any build for the app store strips off the entire bdk library. In that case, we need to update the Xcode symbol strip style.

1. Open Xcode and navigate to your project's settings.
2. Select the "Target Runner" option.
3. In the "Build Settings" tab, locate the "Strip Style".
4. Change the value from "All Symbols" to "Non-Global Symbols."

Creating this issue because I think this should be included in the documentation.

[BitcoinZavior]

Thanks @MrUnfunny Will find a place to include this in docs.
Is there any other issue you encountered when publishing the app to App Store or Play Store?