Update README.md highlighting dependency on sibling plugins

[NicholasSmilovic]

I don't believe it is currently communicated clearly that @nativescript/firebase-core cannot initialize successfully on it's own.

@nativescript/firebase-core initialization code

    const defaultApp = firebase().initializeApp().then(() => {
        console.warn('🔥 Firebase initialized');
      }).catch(error => {
        console.error('🔥 Firebase initialization error: ', error)
      });

fails on android with: Error: Cannot read properties of undefined (reading 'FirebaseApp')
fails on iOS with: Error: FIRApp is not defined

Installing plugin @nativescript/firebase-messaging, I assume any other would work too, makes the initialization a success

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[NicholasSmilovic]

@cla-bot check

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