docs: add CONTRIBUTING guide and link from README

CONTRIBUTING.md

@@ -0,0 +1,92 @@
+# Contributing to webpack-doc-kit
+
+This repository generates webpack’s TypeScript API documentation (Markdown and HTML) from [webpack/webpack](https://github.com/webpack/webpack) `types.d.ts`, using TypeDoc, custom plugins, and [@node-core/doc-kit](https://github.com/nodejs/node-core-utils/tree/main/packages/doc-kit).
+
+Upstream source of truth: **https://github.com/webpack/webpack-doc-kit**. Open pull requests against that repository unless you are intentionally working in a fork.
+
+## Prerequisites
+
+- Node.js (LTS; CI uses `lts/*`)
+- Git
+
+## Local setup
+
+### 1. Clone and install
+
+```bash
+git clone https://github.com/webpack/webpack-doc-kit.git
+cd webpack-doc-kit
+npm ci
+```
+
+If you are using a fork, add the upstream remote so you can stay in sync:
+
+```bash
+git remote add upstream https://github.com/webpack/webpack-doc-kit.git
+```
+
+### 2. Add the webpack sources
+
+The `webpack/` directory is **not** committed; it is listed in `.gitignore`. Generation expects a checkout of **webpack/webpack** at the commit pinned in `HEAD_COMMIT` (same as CI).
+
+**Bash / Git Bash:**
+
+```bash
+git clone https://github.com/webpack/webpack.git webpack
+git -C webpack checkout "$(cat HEAD_COMMIT)"
+```
+
+**PowerShell:**
+
+```powershell
+git clone https://github.com/webpack/webpack.git webpack
+git -C webpack checkout (Get-Content HEAD_COMMIT -Raw).Trim()
+```
+
+### 3. Generate docs and build HTML
+
+```bash
+npm run generate-docs   # TypeDoc → pages/
+npm run build-html      # doc-kit → out/
+```
+
+Full pipeline:
+
+```bash
+npm run build
+```
+
+### 4. Quality checks (same as CI)
+
+```bash
+npm run lint
+npm run format:check
+```
+
+## Before you open a PR
+
+1. Run `npm run lint` and `npm run format:check`.
+2. If you changed anything that affects generated output, run `npm run generate-docs` and commit the updated `pages/` tree when appropriate. CI fails if `pages/` is out of date compared to the pinned webpack commit.
+3. Keep changes focused: `generate-md.mjs`, `plugins/`, themes under `plugins/theme/`, `doc-kit.config.mjs`, and `tsconfig.json` are the usual touch points.
+
+## Google Summer of Code (webpack, 2026)
+
+webpack participates in GSoC. This repo is part of the webpack organization and is relevant if your interests include **API documentation**, **TypeDoc/doc-kit tooling**, or **docs infrastructure**.
+
+**Official onboarding (read this first):**
+
+- Repository: [webpack/gsoc](https://github.com/webpack/gsoc)
+- Project ideas and rules for proposals are published in the doc linked from that README (ideas outside the published list are not accepted).
+- Community: [webpack Discord](https://discord.gg/PebpZRPfJp) (also linked from the GSoC repo).
+
+**Typical GSoC requirement:** one **merged** contribution to the webpack ecosystem before you apply. The [webpack/gsoc README](https://github.com/webpack/gsoc/blob/main/README.md) lists core entry repos (`webpack`, `webpack-cli`, `webpack.js.org`). A merged PR **here** (upstream `webpack/webpack-doc-kit`) is aligned with documentation tooling; if your idea is tied to another repo, confirm with mentors on Discord.
+
+**Practical first steps:**
+
+1. Follow **Local setup** above and ensure `npm run build` succeeds.
+2. Read [webpack’s CONTRIBUTING guide](https://github.com/webpack/webpack/blob/main/CONTRIBUTING.md) for general ecosystem conventions.
+3. Look for `good first issue` / `help wanted` in [webpack-doc-kit issues](https://github.com/webpack/webpack-doc-kit/issues) and related webpack repos.
+
+## License
+
+Licensing follows the [webpack project](https://github.com/webpack/webpack); see upstream repositories for details.

README.md

@@ -47,7 +47,9 @@ This ensures documentation stays in sync with upstream webpack without manual in
 
 ## Contributing
 
-When making changes to documentation generation (plugins, `generate-md.mjs`, `tsconfig.json`), ensure the docs can still be generated successfully. CI will verify this on every pull request.
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for local setup (including how to check out **webpack/webpack** next to this repo), commands that match CI, and notes for **Google Summer of Code** applicants targeting the webpack organization.
+
+In short: after changing generation code, run `npm run generate-docs` and keep `pages/` in sync when CI expects it; pull requests should pass lint, format checks, and the docs freshness check described in `.github/workflows/ci.yml`.
 
 ## License