docs: add contributing guide

CONTRIBUTING.md

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+# Contributing to `@clack/ui`
+
+Thanks for wanting to help! `@clack/ui` is still an early alpha, so APIs change and whole files get rewritten. If you're planning anything bigger than a small fix, open an issue or say hi on [Discord](https://bomb.sh/chat) first. That way nobody loses a weekend to code that's about to move.
+
+## Get set up
+
+Fork the repo, then clone your fork:
+
+```sh
+git clone https://github.com/<your-username>/ui.git
+cd ui
+```
+
+You need the exact Node and pnpm versions listed under `devEngines` in the root `package.json`. This part is picky. The wrong Node stops `pnpm install` right away, and pnpm 11 fails on the `node-pty` build.
+
+Any version manager works. If you use [mise](https://mise.jdx.dev), which is what our lint, format, and test checks use, export `MISE_IDIOMATIC_VERSION_FILE_ENABLE_TOOLS=node,pnpm` and run `mise install` so it reads those versions from `package.json`.
+
+Then install the dependencies:
+
+```sh
+pnpm install
+```
+
+## Commands you'll use
+
+Run these from the repo root. You don't need to build before you run the tests.
+
+| Command                                    | What it does                              |
+| ------------------------------------------ | ----------------------------------------- |
+| `pnpm test`                                | Runs every test                           |
+| `pnpm test packages/ui/src/layout.test.ts` | Runs one test file                        |
+| `pnpm lint`                                | Runs the linter                           |
+| `pnpm format`                              | Formats the code                          |
+| `pnpm format:check`                        | Checks the formatting and changes nothing |
+| `pnpm build`                               | Builds the packages                       |
+| `pnpm bench`                               | Runs the benchmarks                       |
+
+The benchmarks take a few minutes, so you only need them for changes that could affect rendering speed.
+
+## Find your way around
+
+All the code lives in `packages/ui`.
+
+- `src` has the runtime. Start with `createUI` in `src/ui.ts`.
+- Tests sit next to the code as `*.test.ts`.
+- `test` has the test helpers.
+- `bench` has the benchmarks, and its README explains what they measure.
+
+To test what ends up on the screen, use `createTestUI` from `test/ui.ts`. It runs the UI against a fake terminal in the same process, so you can read the screen, type, and press keys:
+
+```ts
+await using ui = await createTestUI({ cols: 12, rows: 2 });
+
+ui.document.documentElement.appendChild(ui.document.createTextNode('hello'));
+
+await expect.poll(() => ui.terminal.screen.getLines()).toEqual(['hello', '']);
+```
+
+Use `expect.poll` here, because the screen doesn't update the moment you change the document.
+
+## Open a pull request
+
+Before you push, make sure these pass:
+
+```sh
+pnpm lint
+pnpm format:check
+pnpm test
+```
+
+CI runs the same three on every pull request, along with the benchmarks and a workflow that publishes a preview build. In your PR, tell us why the change is needed and link the issue if there is one.
+
+## Be kind
+
+Everyone here follows the [Bombshell code of conduct](https://github.com/bombshell-dev/.github/blob/main/CODE_OF_CONDUCT.md). By contributing, you agree that your work is licensed under the [MIT license](./LICENSE).
+
+Stuck on something? Come ask on [Discord](https://bomb.sh/chat).