[iOS] App Check instructions

[Monabr]

Hello.

After installing Firebase Auth on iOS side I see that I can't actually login into the app.

I receive error messages from App Check saying "App is not registered".

I did not setup any checks on my firebase for "App Check" so it is unexpected error.

Please updated README and provide instructions on how to proper setup Firebase Auth on iOS side.

[mirzemehdi]

@Monabr can't reproduce this issue. I think it is related something to Firebase side, and not related to the library

[Monabr]

@mirzemehdi

1. Create new app in firebase
2. Download config
3. Try to log in and do some actions for example upload file to storage

You will be not able to to this because there will be error messages from App Check saying "App is not registered".