Start a document on how to contribute

CONTRIBUTING.md

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+<!--
+SPDX-FileCopyrightText: 2023 Jeff Epler <jepler@gmail.com>
+
+SPDX-License-Identifier: MIT
+-->
+
+# Contributing
+
+Please feel free to submit pull requests or open issues to improve this project.
+
+There's no particular roadmap for chap, it was created to scratch the original developer's itch but has been useful to others as well.
+If you're looking for a way to contribute, check out the open issues.
+If you want to discuss a possible enhancement before beginning work, opening a fresh issue is a good way to start a dialog about your idea.
+
+## Code style & linting
+
+This project uses (pre-commit)[https://pre-commit.com/] to maintain the code's style and perform some quality checks.
+First, install pre-commit: `pip install pre-commit`.
+Then, enable pre-commit via git "hooks" so that (most of the time) pre-commit checks are performed at the time a git commit is made: `pre-commit install`.
+If necessary, you can run pre-commit checks manually for one or more files: `pre-commit run --all` or `pre-commit run --files src/chap/main.py`.
+
+Some tools (e.g., black) will automatically update the working files with whatever changes they recommend.
+Other tools (e.g., pylint) will just tell you what is wrong and require your intervention to fix it.
+It is acceptable to use hints (e.g., `#  pylint: ignore=...` when it's preferable to actually resolving the reported problem
+(e.g., if the report is spurious or a non-standard construct is used for well-considered reasons)
+
+When you create a pull request, `pre-commit run --all` is run in a standardized environment, which occasionally catches things that were not seen locally.
+That green checkmark in github actions is the final arbiter of whether the code is pre-commit clean.