message: Update the CONTRIBUTING.md guidelines so that it uses DCO instead of CLA.
diff: The team behind the [harp.gl](https://github.com/heremaps/harp.gl) gratefully accepts contributions via
  [pull requests](https://help.github.com/articles/about-pull-requests/) filed against the
  [GitHub project](https://github.com/heremaps/harp.gl/pulls). As part of filing a pull request we ask you to
  -accept our [Contributor License Agreement](https://gist.github.com/heremaps-bot/dcb932a07b424f8ed68341054e7e3a53) (CLA)
  -to grant us rights to your contribution.
  +sign off the [Developer Certificate of Origin](https://developercertificate.org/) (DCO).
  In order to maintain a high software quality standard, we strongly prefer contributions to follow these rules:
  