feature: document instance exporting

resolves #154

Disco.Models/Services/Exporting/ExportOptionGroup.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+
+namespace Disco.Models.Services.Exporting
+{
+    public class ExportOptionGroup : List<ExportOptionField>
+    {
+        public string Name { get; set; }
+
+        public ExportOptionGroup(string name)
+        {
+            Name = name;
+        }
+    }
+}