docs: add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,40 @@
+# Contributing
+
+## Setup
+
+You'll need the Node.js version pinned in [`.nvmrc`](.nvmrc) and pnpm, which Corepack provides:
+
+```bash
+nvm install && nvm use
+corepack enable
+pnpm install
+```
+
+Then build, picking one of:
+
+```bash
+pnpm run build      # TypeScript only: enough for most work on the Node.js tooling
+pnpm run bootstrap  # Also builds native artifacts: needs the Android NDK and/or Apple toolchains, and takes a while
+```
+
+Use `pnpm run dev` to rebuild TypeScript on change.
+
+## Before opening a pull request
+
+```bash
+pnpm --filter weak-node-api run prebuild:prepare  # Generates sources that linting needs
+pnpm run lint
+pnpm run prettier:check
+pnpm run depcheck
+pnpm test
+```
+
+`pnpm test` needs a prior `pnpm run bootstrap`. While iterating, run a single package's tests instead, for example `pnpm --filter cmake-rn run test`.
+
+If your change affects a published package, add a changeset with `pnpm changeset`.
+
+## Pull requests
+
+Some CI jobs only run when the pull request has a matching label (`Host 🏡`, `Apple 🍎`, `Android 🤖`, `MacOS 💻`, `Ferric 🦀` or `weak-node-api`). Labels added after a pull request is opened take effect on the next push.
+
+Learn more about the project in the [README](README.md#packages) and in ["how it works"](docs/HOW-IT-WORKS.md).