checklist for reviewers

Currently, as a reviewer approving apps submission, I check the PR against the requirements listed in our [contribution guide](https://github.com/electron/electron-apps/blob/master/contributing.md). The guide is super comprehensive, but a lot of the rules listed there are already automatically checked by [tests](https://github.com/electron/electron-apps/blob/master/test/human-data.js) as part of CI.

It will be very helpful to have a checklist of items that _need_ human eyes & judgment. Better yet, have a bot that comments on each add app PR with said checklist so reviewers can check them off as they go!