docs: Add deprecation message

Author: mediumTaj

src/IBM.Watson.NaturalLanguageUnderstanding.v1/Model/CategoriesOptions.cs

@@ -40,7 +40,10 @@ public class CategoriesOptions
         /// <summary>
         /// Enter a [custom
         /// model](https://cloud.ibm.com/docs/services/natural-language-understanding?topic=natural-language-understanding-customizing)
-        /// ID to override the standard categories model.
+        /// ID to override the standard categories model. The custom categories experimental feature will be retired on 19 December 
+        /// 2019. On that date, deployed custom categories models will no longer be accessible in Natural Language Understanding. 
+        /// The feature will be removed from Knowledge Studio on an earlier date. Custom categories models will no longer be accessible 
+        /// in Knowledge Studio on 17 December 2019.
         /// </summary>
         [JsonProperty("model", NullValueHandling = NullValueHandling.Ignore)]
         public string Model { get; set; }