docs(Assistant V2): Labeled Assistant V2 as private beta

Author: mediumTaj

README.md

@@ -66,7 +66,7 @@ The credentials for each service contain either a `username`, `password` and end
 ## Watson Services
 To get started with the Watson Services in Unity, click on each service below to read through each of their `README.md`'s and their codes.
 * [Assistant V1](/Scripts/Services/Assistant/v1)
-* [Assistant V2](/Scripts/Services/Assistant/v2)
+* [Assistant V2](/Scripts/Services/Assistant/v2) (private beta)
 * [Conversation](/Scripts/Services/Conversation/v1) (deprecated - Use Assistant V1 or Assistant V2)
 * [Discovery](/Scripts/Services/Discovery/v1)
 * [Language Translator V2](/Scripts/Services/LanguageTranslator/v2) (deprecated - Use LanguageTranslator V3)