chore: add GitHub contributions file

CONTRIBUTING.md

@@ -0,0 +1,177 @@
+# Contributing
+
+The project homepage is on
+[GitHub](https://github.com/sr-murthy/continuedfractions).
+
+Contributors and contributions are welcome via pull requests from a fork
+targeting the parent `main`
+[branch](https://github.com/sr-murthy/continuedfractions/tree/main).
+
+A simple Git workflow, using a feature and/or fix branch created off the
+`main` branch of your fork, is recommended.
+
+## Cloning
+
+If you wish to contribute please first ensure you have [SSH access to
+GitHub](https://docs.github.com/en/authentication/connecting-to-github-with-ssh).
+If you do then this should work:
+
+``` bash
+ssh -vT git@github.com
+```
+
+If not please follow the SSH instructions linked above.
+
+Once you’ve forked the repository, you can clone your fork, e.g. over
+SSH:
+
+``` python
+git clone git+ssh://git@github.com/<fork user>/continuedfractions
+```
+
+You can create additional remotes for the parent project to enable
+easier syncing, or you can simply create PRs directly against the parent
+project.
+
+## Dependencies & PDM
+
+The package has no external (production) dependencies - some development
+dependencies are specified in the `[tool.pdm.dev-dependencies]` section
+of the [project
+TOML](https://github.com/sr-murthy/continuedfractions/blob/main/pyproject.toml),
+but they are not mandatory. Of these, the most important are probably
+the `'test'` dependencies, including
+[pytest](https://docs.pytest.org/en/8.0.x/) and
+[pytest-cov](https://pytest-cov.readthedocs.io/):
+
+``` toml
+test = [
+    "coverage[toml]",
+    "pytest",
+    "pytest-cov",
+    "pytest-xdist",
+]
+```
+
+[PDM](https://pdm-project.org/latest) is used (by myself, currently, the
+sole maintainer) to manage all dependencies and publish packages to
+PyPI. It is also used to automate certain tasks, such as running tests,
+as described in the section.
+
+There are no root-level `requirements*.txt` files - but only a single
+(default, version-controlled, cross-platform)
+[pdm.lock](https://github.com/sr-murthy/continuedfractions/blob/main/pdm.lock)
+lockfile, which defines metadata for all TOML-defined development
+dependencies, including the currently empty set of production
+dependencies, and their sub-dependencies etc. This can be used to
+install all development dependencies, including the project itself, in
+editable mode where available:
+
+``` bash
+pdm install -v --dev
+```
+
+> [!NOTE]
+> It is important to note that the `pdm install` uses either the default
+> `pdm.lock` lockfile, or one specified with `-L <lockfile>`. Multiple
+> lockfiles can be generated and maintained. Refer to the [PDM install
+> documentation](https://pdm-project.org/latest/reference/cli/#install)
+> for more information.
+
+If you don't wish to install any editable dependencies, including the
+project itself, you can use:
+
+``` bash
+pdm install -v --dev --no-editable --no-self
+```
+
+The default lockfile can be updated with any and all upstream changes in
+the TOML-defined dependencies, but excluding any editable dependencies
+including the project itself, using:
+
+``` bash
+pdm update -v --dev --no-editable --no-self --update-all
+```
+
+This will usually modify `pdm.lock`, in which case the file should be
+staged and included in a commit.
+
+The lockfile can be exported in its entirety to another format, such as
+`docs/requirements.txt` using:
+
+``` bash
+pdm export -v -f requirements --dev -o docs/requirements.txt
+```
+
+For more information on PDM lockfiles and installing requirements see
+the [PDM documentation](https://pdm-project.org/latest/).
+
+## Makefile and Tests
+
+The [Makefile](Makefile) defines three main targets: `lint` for Ruff
+linting, `doctests` for running
+[doctests](https://docs.python.org/3/library/doctest.html) and
+`unittests` for running unittests and measuring coverage, using `pytest`
+and the `pytest-cov` plugin:
+
+``` bash
+make lint
+make doctests
+make unittests
+```
+
+Linting warnings should be addressed first. The doctests serve as
+acceptance tests, and are best run after the unit tests.
+
+## Documentation
+
+[Project
+documentation](https://continuedfractions.readthedocs.io/en/latest/) is
+defined and built using [Sphinx](https://www.sphinx-doc.org/en/master/),
+and deployed to [Read The Docs](https://readthedocs.org). Currently, the
+building and deployment steps for documentation are not automated in a
+CI pipeline, but are done manually - this will be addressed in future
+releases.
+
+The Sphinx documentation can be built locally on any branch from the
+**project root** using:
+
+``` bash
+make -C docs "html"
+```
+
+First, ensure that you have installed the docs Python requirements,
+which include all development dependencies, either via `pip`:
+
+``` bash
+pip install -r docs/requirements.txt
+```
+
+or via [PDM](https://pdm.fming.dev/latest/):
+
+``` bash
+pdm install -v --dev --no-editable --no-self
+```
+
+## Continuous Integration and Deployment (CI/CD)
+
+The CI/CD pipelines are defined in the [CI
+YML](.github/workflows/ci.yml), and pipelines for all branches include a
+tests stage, consisting of Ruff linting, Python doctests, and unit
+tests, in that order. This will be amended in the future to ensure that
+tests are only run on updates to PRs targeting `main`, to avoid
+duplication on `main`.
+
+## Versioning and Releases
+
+The [PyPI package](https://pypi.org/project/continuedfractions/) is
+currently at version `0.12.5` - the goal is to use [semantic
+versioning](https://semver.org/) consistently for all future releases,
+but some earlier releases do not comply with strict semantic versioning.
+
+There is currently no dedicated pipeline for releases - both [GitHub
+releases](https://github.com/sr-murthy/continuedfractions/releases) and
+[PyPI packages](https://pypi.org/project/continuedfractions) are
+published manually, but both have the same version tag.
+
+Pipelines for releases will be added as part of a future release.