clarify the license and copyright terms of the project

As has been the case, contributions are copyright their respective
contributors and the project is BSD-2 licensed. By contributing to the
project, contributors release their contributions under these copyright
and license terms as declared in LICENSE.
LICENSE governs the whole project so strip file headers
[docs] detail attribution, license, and copyright for development
lint for copyright
Merge pull request #868 from shelhamer/license-copyright

Clarify the license and copyright terms of the project