docs: Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+First, thank you for your interest in contributing to my project. Below is a list of requirements
+that everyone should follow.
+
+1. To avoid wasting your time and effort, please ensure all ideas get discussed first. Either visit
+   [the Ideas discussion board][ideas] and open a thread there, or create a new issue. I ask that
+   you do this to avoid the potential of rejecting work already done in a pull request.
+
+1. **For Markdown changes,** Any and all changes must pass configured [markdownlint] rules (see the
+   `.markdownlint.json` files in this repository for project-specific adjustments to those rules).
+
+1. **For C# changes,** code must conform to the project's style. My day to day coding is done in
+   Jetbrains Rider. If using that IDE, doing a simple [Code Cleanup] on modified source files should
+   be enough. If you're using Visual Studio or some other editor, you are on your own. Formatting
+   rules are stored in `src/.editorconfig` and `src/TrashUpdater.sln.DotSettings`.
+
+[ideas]: https://github.com/rcdailey/trash-updater/discussions/categories/ideas
+[markdownlint]: https://github.com/DavidAnson/markdownlint
+[Code Cleanup]: https://www.jetbrains.com/help/rider/Code_Cleanup__Index.html