Add -zfs option

If blkid can property identify the device as a zfs member this
will get automatically set.  However, older version of blkid were
not ZFS-aware so it's helpful to be able to force zfs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Add basic README.markdown

This is just to get people started, it looks like the full xfstests
documention is also stale and could use an update.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>