Ignore negative level structures in DeReKo#structure.xml

Change-Id: I93a4c13c919c49092edee00a7f0c3d50dbc9a1fe
diff --git a/script/korapxml2krill b/script/korapxml2krill
index ab68d96..5e020df 100644
--- a/script/korapxml2krill
+++ b/script/korapxml2krill
@@ -1143,7 +1143,8 @@
 
 Define the layer for base paragraphs.
 If given, this will be used instead of using C<Base#Paragraphs>.
-Currently C<DeReKo#Structure> is the only additional layer supported.
+Currently C<DeReKo#Structure> and C<DGD#Structure> are the only additional
+layer supported.
 
  Defaults to unset.