README: sbuild+unshare, recommend the faster `zst`, not `xz`
To match the advice at https://wiki.debian.org/sbuild#Option_1:_Using_unshare_with_mmdebstrap_.28no_root_needed.29
To match the advice at https://wiki.debian.org/sbuild#Option_1:_Using_unshare_with_mmdebstrap_.28no_root_needed.29