- Trying to reduce the number of top-level files in Spack.

- CODE_OF_CONDUCT.md can be referenced from the README.

- CONTRIBUTING.md is only used to display a message on pull requests, and
  it still does that when in .github.

We're adopting the contributor covenant for now.  This isn't something we've needed to use yet, but it is good to have in case we actually do have a problem.