Add a .github directory

Add a [CODEOWNERS](https://help.github.com/en/articles/about-code-owners) file. This way @holsee and @swmcc will get informed when a PR has been created.

It should live in a `.github` directory in the root. Feel free to add a [PULL_REQUEST.md](https://help.github.com/en/articles/creating-a-pull-request-template-for-your-repository) as well when there for extra points.

Create the CODEOWNERS file in the repository-root .github directory, assigning @holsee and @swmcc so they are informed when a pull request is created. Optionally add PULL_REQUEST.md there as a pull request template. Done means the files exist in the requested location and GitHub recognizes the ownership and template configuration.

Written by the indexing model from the issue text.