Add prettier instructions to PR template and contribution guide

contributing/how-to-contribute.md

@@ -81,9 +81,10 @@ The core team is monitoring for pull requests. We will review your pull request
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes (`npm test`).
 5. Make sure your code lints (`npm run lint`).
-6. Run the [Flow](https://flowtype.org/) typechecks (`npm run flow`).
-7. If you added or removed any tests, run `./scripts/fiber/record-tests` before submitting the pull request, and commit the resulting changes.
-8. If you haven't already, complete the CLA.
+6. Format your code with [prettier](https://github.com/prettier/prettier) (`npm run prettier`).
+7. Run the [Flow](https://flowtype.org/) typechecks (`npm run flow`).
+8. If you added or removed any tests, run `./scripts/fiber/record-tests` before submitting the pull request, and commit the resulting changes.
+9. If you haven't already, complete the CLA.
 
 ### Contributor License Agreement (CLA)