Add legal files to repo

[derekm]

Add file header template, Apache-2.0 license, and copyright notice.

HEADER and NOTICE files should be made to comply with CNCF-style copyright statements without removing pre-existing Dell copyright notices: https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices

Adopt full customary Apache-2.0 file header template, restoring AS-IS & warranty disclaimer statement.