Add note about sparse files

[Ealdwulf]

The tar implementation used doesn't unpack sparse files correctly, so add a note about this ( we were using bsdtar to generate the archive, and a random file (usr/lib/locale/locale-archive) was for some reason sparse, so it wasn't unpacked currently).

[XECDesign]

Is this still an issue given that we use bsdtar to unpack now?