Add MANIFEST.in to include files in source distro
Add MANIFEST.in to include files in source distro

MANIFEST.in is used to specify files (patterns) to be added to
the source distribution (e.g. on `python setup.py sdist`) in
addition to files that are added by default.

This commit configures MANIFEST.in to add files that are
important/helpful for downstream packagers, i.e. readme, license,
tests and test data and config.

Kudos to @koobs for bringing this to our attention.

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib