Adds a brief CONTRIBUTING onboarding document

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+## Setup
+
+1. Install and configured Python versions however is appropriate for your operating system
+   (try [`pyenv`](https://github.com/pyenv/pyenv)).
+2. Ensure you are working with the latest release version of Poetry, to start.
+   Follow the instructions the installer outputs in order to ensure that `poetry` is on your `PATH`.
+
+       curl -sL https://raw.githubusercontent.com/python-poetry/poetry/master/install-poetry.py | python -
+
+3. Install dependencies
+
+       poetry install
+
+4. Run tests
+
+       poetry run tox
+
+5. If all tests pass for all Python versions, you're in great shape!
+   If tests pass for just one and not the others because you've not installed that Python version, you're
+   probably OK, but keep an eye out for problems when you submit the PR.
+
+6. Install the git pre-commit hooks with [`pre-commit`](https://pre-commit.com/):
+
+       pre-commit install --install-hooks