Document --to-tar option

Change-Id: I92ecae447ad2173cfa2b24082826c7132ef94eb5
diff --git a/Readme.pod b/Readme.pod
index 3b7a5ff..6a9692e 100644
--- a/Readme.pod
+++ b/Readme.pod
@@ -299,6 +299,13 @@
 network latency.
 
 
+=item B<--to-tar>
+
+Only valid for the C<archive> command.
+
+Writes the output into a tar archive.
+
+
 =item B<--sigle|-sg>
 
 Extract the given texts.