CLOUDP-185079: notify users about new SDK (#499)

Co-authored-by: Sarah Simpers <82042374+sarahsimpers@users.noreply.github.com>

Author: wtrocki

README.md

@@ -6,6 +6,12 @@ A Go HTTP client for the [MongoDB Atlas API](https://docs.atlas.mongodb.com/api/
 
 Note that `go-client-mongodb-atlas` only supports the two most recent major versions of Go.
 
+## Maintenance 
+
+`go-client-mongodb-atlas` is targeting Atlas API V1 and v1.5.
+For new projects, please consider the official [Atlas Go SDK](https://github.com/mongodb/atlas-sdk-go)
+To learn how to migrate from go-client-mongodb-atlas to the Atlas Go SDK, see the [migration guide](https://github.com/mongodb/atlas-sdk-go/blob/main/docs/doc_3_migration.md).
+
 ## Usage
 
 ```go