Add CONTRIBUTING.md.

CONTRIBUTING.md

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+# Contributing
+
+Interested in contributing? Great! Here are some suggestions to make it a good
+experience:
+
+Start by [opening an issue][github-issues], whether to identify a problem or
+suggest a change. That issue should be used to discuss the situation and agree
+on a plan of action before writing code or sending a pull request. Maybe the
+problem isn't really a problem, or maybe there are other things to consider. If
+so, it's best to realize that before spending time and effort writing code that
+may not get used.
+
+Match the coding style of the files you edit. Although everyone has their own
+preferences and opinions, a pull request is not the right forum to debate them.
+
+Package versions for `dependencies` and `devDependencies` should be specified
+exactly (also known as "pinning"). The short explanation is that doing otherwise
+eventually leads to inconsistent behavior and broken functionality. (See [Pin
+your npm/yarn dependencies][pin-dependencies] for a longer explanation.)
+
+Add tests for all new/changed functionality. Test positive and negative
+scenarios. Try to break the code now, or else it will get broken later.
+
+Run local tests via `npm test`. Lint by running `npm run lint`. The best way to
+test GitHub Actions is via Workflows, so create a (draft) PR to see your changes
+in action.
+
+Pull requests should contain a single commit that addresses a single issue.
+
+Open pull requests against the `main` branch. Include the text "(fixes #??)." at
+the end of the commit message so it will be associated with the corresponding
+issue. (See [the GitHub documentation][linking-pull-request] for details.)
+
+Please refrain from using slang or meaningless placeholder words. Sample content
+can be "text", "code", "heading", or the like. Sample URLs should use
+[example.com][example-com] which is safe for this purpose. Profanity is not
+allowed.
+
+In order to maintain the permissive MIT license this project uses, all
+contributions must be your own and released under that license. Code you add
+should be an original work and should not be copied from elsewhere. Taking code
+from a different project, Stack Overflow, or the like is not allowed. The use of
+tools such as GitHub Copilot, ChatGPT, LLMs (large language models), etc. that
+incorporate code from other projects is not allowed.
+
+Thank you!
+
+[example-com]: https://en.wikipedia.org/wiki/Example.com
+[github-issues]: https://github.com/DavidAnson/markdownlint-cli2/issues
+[linking-pull-request]: https://docs.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword
+[pin-dependencies]: https://maxleiter.com/blog/pin-dependencies