Document the use of tar.xz files

When downloading the Tor bundle for Debian today I got a 'tar.xz' file. However the help page located at https://www.torproject.org/docs/tor-doc-unix.html.en#using only explains how to deal with a 'tar.gz' file.

Maybe you should add here the 'xJf' tar option needed to deal with the 'tar.xz' extension?

Trac:
Username: LilKS