Update README.md to include links for Migration Doc (#3573)

* Update README.md to include links for Migration Doc

* Update README.md

Changed to lower case `v2`

Co-authored-by: Matt Creaser <[email protected]>

---------

Co-authored-by: Matt Creaser <[email protected]>

Author: yuhengshs

README.md

@@ -4,6 +4,8 @@
 [![GitHub release](https://img.shields.io/github/release/aws-amplify/aws-sdk-android.svg)](https://github.com/aws-amplify/aws-sdk-android/releases)
 [![Maven Central](https://img.shields.io/maven-central/v/com.amazonaws/aws-android-sdk-core.svg)](https://search.maven.org/search?q=a:aws-android-sdk-core)
 
+For existing customers planning to migrate to the new Amplify Android v2, we recommend reviewing our [migration guide](https://github.com/aws-amplify/amplify-android/blob/main/documents/MobileSDK_To_AmplifyAndroid.md).
+
 For new projects, we recommend using the latest v2 version of [AWS Amplify Library for Android](https://docs.amplify.aws/start/q/integration/android/) to quickly implement common app use cases like Authentication, Storage, Push Notifications and more.
 
 Note: v2 of Amplify Library for Android (currently GA) is built on top of [the AWS SDK for Kotlin](https://aws.amazon.com/sdk-for-kotlin/). You can access this underlying SDK via [the Escape Hatch from AWS Amplify](https://docs.amplify.aws/lib/project-setup/upgrade-guide/q/platform/android/#escape-hatches).