docs: provide developer guidance

CONTRIBUTING.md

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+# Contributing
+
+This project uses the [Git feature branch workflow](https://www.atlassian.com/git/tutorials/comparing-workflows/feature-branch-workflow).  Please submit your changes for review as a [GitHub pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests).
+
+## Development Environment
+
+This project requires Python 3.10 or newer.  To set up your development environment on Linux, run these commands from the project root directory:
+
+- `sudo make build-deps`—installs build dependencies (Debian/Ubuntu only)
+
+- `make`—creates a virtual environment named `.venv` in the current working directory and performs an editable installation of this project, including development and testing tools
+
+- `make pre-commit`—installs pre-commit hooks (requires the virtual environment to be active in your code editor or [Git porcelain](https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain))
+
+- `make test`—performs comprehensive functional and integration testing of this project
+
+- `make smoke`—runs a shorter, faster subset of the test suite
+
+- `make docker`—builds a fully tested and release-ready container image
+
+Additional [make(1)](https://linux.die.net/man/1/make) targets are available.  Review the [Makefile](Makefile) for details.
+
+## Code Style
+
+The following code styles are in use:
+
+- [Python Black](https://black.readthedocs.io/) and [isort](https://pycqa.github.io/isort/)
+
+- [Dockerfile best practices](https://docs.docker.com/develop/develop-images/dockerfile_best-practices/)
+
+- [the Home Assistant YAML style guide](https://developers.home-assistant.io/docs/documenting/yaml-style-guide/)
+
+## Commit Messages
+
+This project implements [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html) using [Conventional Commits 1.0.0](https://www.conventionalcommits.org/en/v1.0.0/).  Valid commit types are:
+
+- **build**—changes to the build system or external dependencies
+
+- **ci**—changes to the CI configuration files and scripts
+
+- **docs**—documentation-only changes
+
+- **feat**—a new feature
+
+- **fix**—a bug fix
+
+- **perf**—a code change that improves performance
+
+- **refactor**—a code change that neither fixes a bug nor adds a feature
+
+- **test**—new tests or corrections to existing tests
+
+A commit's scope should be the second-level Python module name sans the `stuart.` prefix or any suffixes with a few exceptions.  Valid commit scopes are:
+
+- [stuart](src/__init__.py)—for the corresponding top-level dunder module
+
+- [app](src/app.py)—the web application, using [Flask](https://flask.palletsprojects.com/)
+
+- [main](src/main.py)—the main web page and related routes
+
+- [wsgi](src/wsgi.py)—the [WSGI adapter](https://en.wikipedia.org/wiki/Web_Server_Gateway_Interface), for use with [gunicorn](https://gunicorn.org/)
+
+- **packaging**—package layout or other metadata, e.g., the arrangement of [src/](src/), alterations to [pyproject.toml](pyproject.toml) or [Dockerfile](Dockerfile)
+
+- no scope—for **refactor** or **test** changes covering multiple packages; or for **build**, **ci**, or **doc** changes not specific to one package