docs(readme): add reason for deprecation

Author: kevinkowa

README.md

@@ -89,6 +89,12 @@ If you have more than one plan, you can use `CredentialUtils` to get the service
 
 Watson services are migrating to token-based Identity and Access Management (IAM) authentication.
 
+As of `v9.2.1` the preferred approach of initializing an authenticator is by using the builder pattern. 
+The main reason is to give us flexibility to add new properties to the authenticator in the future if and when needed, 
+and to make it easier for users to construct an authenticator instance using the precise set of properties they need to 
+set for their use-case rather than being forced to use one of the multi-arg constructors (i.e. the one that most closely 
+matches their requirements).
+
 - You can initialize the authenticator with either of the following approaches:
     - In the builder of the authenticator (builder pattern).
     - In the constructor of the authenticator (deprecated, but still available).