File .github/mergeable.yml from the latest check-in

version: 2
mergeable:
  - when: pull_request.opened
    name: "Greet a contributor"
    validate: []
    pass:
      - do: comment
        payload:
          body: >
            Thanks for your contribution!  Unfortunately, we don't use GitHub pull requests to manage code contributions to this repository.  Instead, please see the [REDUCE Homepage](https://reduce-algebra.sourceforge.io/) for details on how to get involved with REDUCE.
      - do: close