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 [GRG Homepage](https://reduce-algebra.sourceforge.io/grg32/grg32.php), the [REDUCE Homepage](https://reduce-algebra.sourceforge.io) or the [REDUCE Support Page](https://sourceforge.net/projects/reduce-algebra/support) at Sourceforge for further details."
      - do: close