Explain level prefixing

Change-Id: Ib2b422ee5ade99af4babbc1ab272cc32bb4ca413
diff --git a/Readme.pod b/Readme.pod
index 011caa6..70755ba 100644
--- a/Readme.pod
+++ b/Readme.pod
@@ -553,7 +553,9 @@
 
 =item C<name>
 
-The key of the meta object
+The key of the meta object, that may be prefixed by C<corpus> or C<doc>, in case the
+C<E<lt>xenoDataE<gt>> information is located on these levels. The text level introduces
+no prefixes.
 
 =item C<project> (optional)