chore: updates README to clarify deprecation. (#529)

Co-authored-by: Massimiliano Marcon <massimiliano.marcon@mongodb.com>

README.md

@@ -6,10 +6,10 @@ 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 
+## This package is deprecated
+
+**The `go-client-mongodb-atlas` is deprecated** and doesn't receive feature updates. We recommend migrating to the [Atlas Go SDK](https://github.com/mongodb/atlas-sdk-go) for the latest changes.
 
-`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