Add docs file for each package.

Add docs file for each package.

Resolve this lintian warning:

W: zfsutils: extra-license-file usr/share/doc/zfsutils/COPYING
N:
N:    All license information should be collected in the debian/copyright
N:    file. This usually makes it unnecessary for the package to install this
N:    information in other places as well.
N:
N:    Refer to Debian Policy Manual section 12.5 (Copyright information) for
N:    details.
N:
N:    Severity: normal, Certainty: possible
N: