Add migration advisory to readme and migration guide

MIGRATION_GUIDE.md

@@ -1,5 +1,16 @@
 # React Native v5.0.0 Migration Guide
 
+#### ⚠️ Migration Advisory for current OneSignal customers
+
+Our new [user-centric APIs and v5.x.x SDKs](https://onesignal.com/blog/unify-your-users-across-channels-and-devices/) offer an improved user and data management experience. However, they may not be at 1:1 feature parity with our previous versions yet.
+
+If you are migrating an existing app, we suggest using iOS and Android’s Phased Rollout capabilities to ensure that there are no unexpected issues or edge cases. Here is the documentation for each:
+
+- [iOS Phased Rollout](https://developer.apple.com/help/app-store-connect/update-your-app/release-a-version-update-in-phases/)
+- [Google Play Staged Rollouts](https://support.google.com/googleplay/android-developer/answer/6346149?hl=en)
+
+If you run into any challenges or have concerns, please contact our support team at support@onesignal.com
+
 # Intro
 
 In this release, we are making a significant shift from a device-centered model to a user-centered model. A user-centered model allows for more powerful omni-channel integrations within the OneSignal platform.

README.md

@@ -8,6 +8,19 @@
 
 ---
 
+#### ⚠️ Migration Advisory for current OneSignal customers
+
+Our new [user-centric APIs and v5.x.x SDKs](https://onesignal.com/blog/unify-your-users-across-channels-and-devices/) offer an improved user and data management experience. However, they may not be at 1:1 feature parity with our previous versions yet.
+
+If you are migrating an existing app, we suggest using iOS and Android’s Phased Rollout capabilities to ensure that there are no unexpected issues or edge cases. Here is the documentation for each:
+
+- [iOS Phased Rollout](https://developer.apple.com/help/app-store-connect/update-your-app/release-a-version-update-in-phases/)
+- [Google Play Staged Rollouts](https://support.google.com/googleplay/android-developer/answer/6346149?hl=en)
+
+If you run into any challenges or have concerns, please contact our support team at support@onesignal.com
+
+---
+
 [OneSignal](https://onesignal.com/) is a free email, sms, push notification, and in-app message service for mobile apps. This SDK makes it easy to integrate your native React-Native iOS and/or Android apps with OneSignal.
 
 #### Installation