Add buchhaltung email as contact on invoice

Change-Id: Iada8a51bfe68f9ee48a8cee59dd2e90bea6f9bf9
diff --git a/invoice.php b/invoice.php
index 267b603..841a772 100644
--- a/invoice.php
+++ b/invoice.php
@@ -82,6 +82,8 @@
 Reference: ICLC-10-" . $user->id . "
 
 VAT ID: DE 143 845 359
+
+If you have any questions about this invoice, please contact us via buchhaltung@ids-mannheim.de.
 ");
     //$invoice->setFooterNote("xx");