docs: write a CONTRIBUTING.md

[Anomalocaridid]

I do not know how interested other people are in contributing to this, but regardless, it would be helpful to document what tools are used, such as cargo-mutants, whether for potential contributors, or just reminding myself how to do things in the event I take a long break from this project.

Currently, the only departure from the "standard" Rust workflow is using cargo-mutants to measure test coverage.

The use of conventional commits is also something that should be documented.