Documents: Add CreditNote

Elwig/Documents/Document.cshtml.cs

@@ -49,6 +49,8 @@ namespace Elwig.Documents {
                 name = "BusinessLetter";
             } else if (this is DeliveryNote) {
                 name = "DeliveryNote";
+            } else if (this is CreditNote) {
+                name = "CreditNote";
             } else {
                 throw new InvalidOperationException("Invalid document object");
             }