Update CONTRIBUTING.md

[hwayne]

Add a template for submitters. Doing this as a PR to check that the checklist functionality works.

• [ ] I have followed the code of conduct.
• [ ] I have followed the contributing guidelines and used the proper item format.
• [ ] This is not a duplicate.
• [ ] The resource is free.
• [ ] The resource is rigorous because: (please list)
• [ ] The resource is not obnoxious, condescending, etc.
• [ ] My writup is respectful of both sides.
• [ ] I've added reasonable caveats.
• [ ] I've noted if the resource is a video, pdf, etc.