README: Change gzip/bzip2 to xz compression format

Kernel.org is only hosting patches and kernel compressed with xz,
so change the old gzip/bzip2 instances to xz.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed