Add CONTRIBUTING.md

[seriouslysean]

Maybe outline any sort of code quality concerns, etc in this file to set expectations for contributors?

• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file#supported-file-types
• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

I really like the open government template, might be worth looking at as an example, https://github.com/opengovernment/opengovernment/blob/master/CONTRIBUTING.md.

[itsalaidbacklife]

Closed by #165