Document contributor setup

[Mirochill]

Summary

• document Poetry as the current development setup instead of the historical tox config
• describe the local PostgreSQL access expected by the test fixtures
• list the common lint, test, and pre-commit commands used by the repo

Validation

• git diff --check
• git diff --cached --check
• git show --check --stat --oneline HEAD
• CircleCI lint: failed while installing the pre-commit hook environment because a dependency metadata check rejects tool.poetry.extras.pipfile_deprecated_finder[2]
• CircleCI build-then-publish: PostgreSQL jobs fail in tests/test_all.py::test_weird_names during sqlbag.temporary_database() setup with SQLAlchemy ArgumentError: List argument must consist only of dictionaries
• Not run locally.

Closes #35