chore: update README about the repository relocation (#630)

README.md

@@ -7,6 +7,17 @@ Java protobuf classes for IAM Policy.
 
 - [Client Library Documentation][javadocs]
 
+:bus: In October 2022, this library has moved to
+[google-cloud-java/java-accessapproval](
+https://github.com/googleapis/google-cloud-java/tree/main/java-iam).
+This repository will be archived in the future.
+Future releases will appear in the new repository (https://github.com/googleapis/google-cloud-java/releases).
+The Maven artifact coordinates (`com.google.cloud:google-iam-policy`) remain the same.
+
+The other artifacts (`grpc-google-iam-v1`, `grpc-google-iam-v2`, `grpc-google-iam-v2beta`,
+`proto-google-iam-v1`, `proto-google-iam-v2`, and `proto-google-iam-v2beta`) have moved to
+[sdk-platform-java/java-iam](https://github.com/googleapis/sdk-platform-java/tree/main/java-iam).
+
 ## Java Versions
 
 Java 7 or above is required for using this client.