Add 'What is ZFS' section

Based on the comments in https://github.com/zfsonlinux/zfs/issues/1397
a basic 'What is ZFS' section was added.  Improvements to this section
are always welcome.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#1397