Make Archive::Tar::Builder optional

Change-Id: I557617b0884b0b30d70c54003a3afda8c1a6b0e3
diff --git a/Readme.pod b/Readme.pod
index b7445ab..50b9165 100644
--- a/Readme.pod
+++ b/Readme.pod
@@ -28,7 +28,8 @@
 In case everything went well, the C<korapxml2krill> tool will
 be available on your command line immediately.
 Minimum requirement for L<KorAP::XML::Krill> is Perl 5.16.
-Optional support for L<Sys::Info> to calculate available cores.
+Optionally installing L<Archive::Tar::Builder> speeds up archive building.
+Optional support for L<Sys::Info> to calculate available cores is available.
 In addition to work with zip archives, the C<unzip> tool needs to be present.
 
 =head1 ARGUMENTS