Add LICENSE, NOTICE, and CONTRIBUTING docs

This pull request licenses the project under the Apache License 2.0.

The Apache License 2.0 is a permissive license like MIT, with two additional benefits:

1. All contributions made to the project are automatically licensed under the same terms.
2. Contributors waive any patent rights to contributions.

This makes it easy and safe to accept PRs on GitHub. (Something not many people think about.)

I have included three top-level docs:

`LICENSE.md` is a place to note the licenses that apply to the code and any third-party modules.

`NOTICE.md` is a place to note copyright notices and other legal notices.

`CONTRIBUTING.md` makes sure that people submitting PRs are aware that contributions to the project are automatically licensed under the terms of the Apache License 2.0.