chore: add check to PR template for exports (elastic#193)

.github/pull_request_template.md

@@ -8,6 +8,7 @@ If it involves visual changes include a screenshot or gif.
 
 Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.
 
+- [ ] Any consumer-facing exports were added to `src/index.ts` (and stories only import from `../src` except for test data & storybook)
 - [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)
 - [ ] Proper documentation or storybook story was added for features that require explanation or tutorials
 - [ ] Unit tests were updated or added to match the most common scenarios