Add CLA callout to PR template

Author: interactivellama

docs/PULL_REQUEST_TEMPLATE.md

@@ -7,6 +7,7 @@ Fixes #
 ### CONTRIBUTOR checklist (do not remove)
 Please complete for every pull request
 
+* [ ] First-time contributors should sign the Contributor License Agreement. It's a fancy way of saying that you are giving away your contribution to this project. If you haven't before, wait a few minutes and a bot will comment on this pull request with instructions.
 * [ ] `npm run lint:fix` has been run and linting passes.
 * [ ] Mocha, Jest (Storyshots), and `components/component-docs.json` CI tests pass (`npm test`).
 * [ ] Tests have been added for new props to prevent regressions in the future. See [readme](https://github.com/salesforce/design-system-react/blob/master/tests/README.md).