docs: Update readme to add GA4 information (#18)

Author: rmi22186

README.md

@@ -1,3 +1,5 @@
+> Google launched Google Analytics 4, a new way of analyzing iOS, Android, and Web data together.  Google encourages all customers to [upgrade their Firebase instances](https://support.google.com/analytics/answer/9379599) so that Firebase data flows to Google Analytics 4.  mParticle encourages all customers to also upgrade to the [new mParticle iOS Firebase GA4 kit](https://github.com/mparticle-integrations/mparticle-apple-integration-google-analytics-firebase-ga4). This repository page you are on will continue to work, but all new code changes and bug fixes will be made in our new Firebase GA4 kit.  Please see the [mParticle GA4](https://docs.mparticle.com/integrations/google-analytics-4/event) docs for more information on upgrading.
+
 ## Google Analytics for Firebase Kit Integration
 
 This repository contains the [Google Analytics for Firebase](https://firebase.google.com/) integration for the [mParticle Apple SDK](https://github.com/mParticle/mparticle-apple-sdk).