Restore a pull request template entry

Follow up to a7af7bc#r61575823.

Author: koic

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,5 +15,6 @@ Before submitting the PR make sure the following are checked:
 * [ ] Added an entry (file) to the [changelog folder](https://github.com/rubocop/rubocop-rails/blob/master/changelog/) named `{change_type}_{change_description}.md` if the new code introduces user-observable changes. See [changelog entry format](https://github.com/rubocop/rubocop/blob/master/CONTRIBUTING.md#changelog-entry-format) for details.
 * [ ] The PR relates to *only* one subject with a clear title
   and description in grammatically correct, complete sentences.
+* [ ] If this is a new cop, consider making a corresponding update to the [Rails Style Guide](https://github.com/rubocop/rails-style-guide).
 
 [1]: https://chris.beams.io/posts/git-commit/