Add a PR template to point to CONTRIBUTING.md

[marcofranssen]

To ensure contributors have read the CONTRIBUTING.md it would be good to have a PR template that adds following at a minimum:

Please describe the changes of your PR here.

- [ ] I have read the [CONTRIBUTING.md](CONTRIBUTING.md) and comply with those guidelines.
- [ ] I have added tests

Any suggestions on making the PR template better are welcome.