Return NOMOREDOCS constant on skip_doc

Change-Id: I65a9511c29cbac342f00f0dda0cb2206215ddfee
diff --git a/lib/Krawfish/Corpus/Distribution.pm b/lib/Krawfish/Corpus/Distribution.pm
index 6120a1a..4b84c32 100644
--- a/lib/Krawfish/Corpus/Distribution.pm
+++ b/lib/Krawfish/Corpus/Distribution.pm
@@ -25,7 +25,10 @@
 #
 #   However - in case skips are not available,
 #   this may be slow ...
-#
+
+# TODO:
+#   Support corpus classes
+
 # TODO:
 #   Another distribution variant would be
 #   distr(doc[1:1]: 'author:Goethe', 'author:Schiller')