add guidelines to creating notebooks to be tested on the CI

[atqy]

Issue #, if available:

Description of changes: Add additional guidelines to CONTRIBUTING to make notebook friendly for testing on CI

Testing done: None needed

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• [x] I have read the CONTRIBUTING doc and adhered to the example notebook best practices
• [x] I have updated any necessary documentation, including READMEs
• [ ] I have confirmed secret codes are not included in the example.
• [ ] I have tested my notebook(s) and ensured it runs end-to-end
• [ ] I have linted my notebook(s) and code using black-nb -l 100 {path}/{notebook-name}.ipynb

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.