README: sparse image creation

libguestfs-tools package is nessesary to make image
sparse.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Sparse file explaination for several distributions
Merge pull request #25 from miurahr/sparse-disk-image

README: sparse image creation
README: ubuntu PPA for 12.04
Merge pull request #34 from miurahr/readme-ubuntu-support

merged README: ubuntu PPA for 12.04