Add a reviewer checklist to the PR template

[laurenfrederick]

Description

Add a reviewer checklist to the PR template

Issue number or link

Author checklist

• I have submitted a Contributor License Agreement (only needed once). - N/A
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes). - N/A
• I have added or updated unit tests to ensure consistent code coverage as necessary. - N/A
• I have updated the documentation as necessary. - N/A

[j9liu]

Thanks @laurenfrederick !