message: Add DCO to contribution guidelines
diff: @@ -22,6 +22,10 @@ The minimum requirements for code contributions are:
  3. New and changed challenges _must_ have a corresponding e2e test.
  4. Linting, as well as all unit, integration and e2e tests _should_ pass
  locally before opening a Pull Request.
  +5. All Git commits within a PR must be
  + [signed off](https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--s)
  + to indicate the contributor's agreement with the
  + [Developer Certificate of Origin](https://developercertificate.org/).
  ### Contribution Guidelines
  