Proposal: Get rid of copyright headers on each file

There are a few problems with the copyright headers on each file
* It's annoying to drive-by contributors to put their names on each file.
* It's annoying to folks who then come and change that file to also put their names on the file.
* As the files get more contributors, you then have a long list of copyright owners. Annoying.

None of this is necessary. [Apache](https://www.apache.org/dev/apply-license.html) says.
> Each original source document (code and documentation, but excluding the LICENSE and NOTICE files) SHOULD include a short license header at the top. If the distribution contains documents not covered by CLA, CCLA or Software Grant (such as third-party libraries) then see the policy guide."

I propose we get rid of the copyright headers on each file and instead have a NOTICE.md file that has an alphabetical list of names of contributors. The contribution guide will encourage contributors to add their names. And regardless of whether or not they choose to add their names, they still keep copyright (at least in US copyright law). And if we ever want to know who wrote what class, that's what `git blame` is for.

Review the proposal and its Apache licensing reference first. Identify all affected copyright headers, then assess the proposed NOTICE.md and contribution guide changes; done means the project’s policy is agreed, headers are consistently handled, and contributor-name guidance is documented.

Written by the indexing model from the issue text.