pain: added Document root element placeholder

pain/pain.go

@@ -5,6 +5,14 @@ import (
 	"strings"
 )
 
+type Document struct {
+	xsi       string  `xml:"xmlns:xsi,attr"`
+	namespace string  `xml:"xmlns,attr"`
+	Contents  PainXML `xml:"CstmrDrctDbtInitn"`
+}
+
+// TODO fill in namespace and xsi
+
 type PainXML struct {
 	GroupHeader            GrpHdr   `xml:"GrpHdr"`
 	PaymentInformation     []PmtInf `xml:"PmtInf"`