Add CONTRIBUTING file

[jacob-rosenthal]

The CONTRIBUTING file gives instructions and guidelines for contributors. We can start with something basic and expand as needed down the road.

There is a contributing.rst file in the documentation, but we should have it in the root directory so it is easily accessible. Also, if it is in the root directory, GitHub will automatically link to this file when a contributor creates an issue or opens a pull request.

Helpful resources:

• http:https://opensource.guide/starting-a-project/#writing-your-contributing-guidelines
• https://mozillascience.github.io/working-open-workshop/contributing/
• https://github.com/nayafia/contributing-template/blob/HEAD/CONTRIBUTING-template.md