docs: update treewide documentation

CONTRIBUTING.md

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-A guide to how you can contribute to proselint can be found on [our website].
+# Contributing
 
-Work on proselint happens over at [our GitHub]. To contribute, create a new
-branch off main and then open a pull request. Your code should run cleanly
-through `pycodestyle` and `pydocstyle` linters (pro tip \- outside of CI,
-use `pycodestyle --config=pyproject.toml && pydocstyle -se` for more
-descriptive information on what's causing your lint failures). Comments,
-bug reports, and other feedback can be provided through GitHub issues.
+Interested in contributing to `proselint`? Great — there are plenty of ways you
+can help. In the following sections, we describe how you can help us build
+`proselint` into the greatest writing tool in the world.
 
-[our website]: http://proselint.com/contributing
-[our GitHub]: http://github.com/amperser/proselint
+## Skills
+
+### Coding
+
+Have coding skills? You can:
+
+- Implement a new piece of advice in the linter. In our issue tracker on GitHub,
+  we maintain an ever-growing [list of rules] that have been extracted from
+  expert advice on writing. Some of these are one-liners that will take only a
+  moment to complete; others are full-blown research projects. Use existing
+  checks as a guide for creating your own. Then open a pull request.
+- Fix [bugs].
+- Improve [performance].
+- Write tests. Our test coverage is too low. Keeping out false alarms is key to
+  the tool's success and anything we can do to uncover false alarms and prevent
+  regressions is thus key to the tool's success.
+
+[list of rules]: https://github.com/amperser/proselint/labels/cat:%20new-check
+[bugs]: https://github.com/amperser/proselint/labels/type:%20fix
+[performance]: https://github.com/amperser/proselint/labels/type:%20perf
+
+### Writing and editing
+
+Have writing or editing skills? You can:
+
+- Extract rules from expert sources of advice. Read through some entries of a
+  book like *Garner's Modern American usage*, find rules that are feasible to
+  implement, and then [open a Check Request issue]. Here's an [example] of a
+  successfully extracted rule.
+- Find false positives. Run `proselint` over your favourite books or magazine
+  articles — maybe even your own writing — and find places where `proselint`
+  says there's an error, but in reality, there is none. [Open a False Positive
+  issue] and describe the problem.
+- Improve the copy or documentation surrounding the project and within its code.
+
+[open a Check Request issue]: https://github.com/amperser/proselint/issues/new?template=check-request.md
+[example]: https://github.com/amperser/proselint/issues/163
+[Open a False Positive issue]: https://github.com/amperser/proselint/issues/new?template=false-positive.md
+
+## Process
+
+### Prerequisites
+
+Before you spend valuable time contributing to Proselint, please first discuss
+the change you wish to make with us. The best way to do this is by filing an
+issue with your proposal, since you'll need an issue number to refer to in your
+pull request. Alternatively, you can skip this if an issue relating to your
+changes already exists.
+
+Next, ensure your environment is prepared. If you're using [Nix], this is as
+simple as running `nix develop`. For other systems, you'll need to install
+[`uv`], and then run `uv sync --locked --dev --all-extras --group test` for the
+full suite of development and testing tools.
+
+[Nix]: https://nixos.org
+[`uv`]: https://docs.astral.sh/uv
+
+### Workflow
+
+1. Fork and clone this repository.
+2. Create a new branch in your fork based off the **main** branch.
+3. Make your changes.
+4. Commit your changes, and push them.
+5. Submit a pull request.
+
+### Linting and Testing
+
+We use [Poe] as a job runner. This makes the process of linting and testing your
+changes in the same way they'll be verified in CI trivial. Please do ensure
+these pass before you submit a pull request to keep your changes consistent with
+our codebase.
+
+- **Linting**: `uv run poe lint`
+- **Formatting**: `uv run poe format`
+- **Testing**: `uv run poe test`
+
+[Poe]: https://poethepoet.natn.io