add comments about model

Author: timt-unity3d

Analyzer/SQLite/Parsers/Models/BuildLayout.cs

@@ -77,6 +77,9 @@ public class Reference : ReferenceId
         public ReferenceType type;
     }
 
+    // This is a grab bag of data because Unity serialization does not 
+    // support polymorphism, so all possible fields are listed in the
+    // ReferenceData class.
     public class ReferenceData
     {
         public ReferenceId Bundle;