housekeeping: Add none option to RegistrationNamespaces

Author: glennawatson

src/ReactiveUI/RegistrationNamespace.cs

@@ -10,6 +10,9 @@ namespace ReactiveUI
     /// </summary>
     public enum RegistrationNamespace
     {
+        /// <summary>No platform to register.</summary>
+        None = 0,
+        
         /// <summary>
         /// Xamarin Forms.
         /// </summary>