Add migration advisory to readme and migration guide

Author: jennantilla

MIGRATION_GUIDE.md

@@ -1,5 +1,15 @@
 # Flutter 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 [email protected]
 
 # Intro
 

README.md

@@ -6,6 +6,19 @@
 
 ---
 
+#### ⚠️ 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 [email protected]
+
+---
+
 [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 Flutter iOS and/or Android apps with OneSignal.
 
 <p align="center"><img src="https://app.onesignal.com/images/android_and_ios_notification_image.gif" width="500" alt="Flutter Notification"></p>