Update PULL_REQUEST_TEMPLATE.md
* Update PULL_REQUEST_TEMPLATE.md
noticed these items in the chef pr template, and think it would be good to have them in automate too
* Update .github/PULL_REQUEST_TEMPLATE.md
### :white_check_mark: Checklist
+- [ ] I have read the [CONTRIBUTING document](../CONTRIBUTING.md).
- [ ] Tests added/updated?
- [ ] Docs added/updated?
+- [ ] All commits have been signed-off for the [Developer Certification of Origin](https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco).
### :camera: Screenshots, if applicable
