doc: document the GN build

doc/contributing/gn-build.md

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+# GN Build
+
+Similar to GYP, GN is a build system designed for building Chromium. The
+official builds of Node.js are built with GYP, but GN build files are also
+provided as an unofficial alternative build system.
+
+Currently the GN build is used by:
+
+1. Electron for building Node.js together with Chromium.
+2. V8 for testing the integration of Node.js in CI.
+
+## Files of GN
+
+Node.js contains following GN build files:
+
+* `node.gni` - Public GN arguments for configuring the build.
+* `*/BUILD.gn` - This is the file where GN looks for build rules, in Node.js it
+  just includes the `unofficial.gni` file to avoid accidental modifications.
+* `*/unofficial.gni` - The actual build rules for each component.
+
+## Build with GN
+
+Unlike GYP, the GN tool does not include any built-in rules for compiling a
+project, which means projects building with GN must provide their own build
+configurations for things like how to invoke a C++ compiler. Chromium related
+projects like V8 and skia choose to reuse Chromium's build configurations, and
+we are reusing V8's Node.js integration testing repository
+([node-ci](https://chromium.googlesource.com/v8/node-ci/)) for building Node.js.
+
+### 1. Install `depot_tools`
+
+The `depot_tools` is a set of tools used by Chromium related projects for
+checking out code and managing dependencies, and since we are reusing the infra
+of V8, we have to install and add it to `PATH`:
+
+```bash
+git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
+export PATH=/path/to/depot_tools:$PATH
+```
+
+You can also follow the [official tutorial of
+`depot_tools`](https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html).
+
+### 2. Check out code
+
+To check out the latest main branch of Node.js for building, use the `fetch`
+tool from `depot_tools`:
+
+```bash
+mkdir node_gn
+cd node_gn
+fetch node
+```
+
+You can choose to save some time by omitting git history:
+
+```bash
+fetch --no-history node
+```
+
+After syncing is done, you will get a directory structure like this:
+
+```console
+node_gn/
+├── .gclient
+├── .gclient_entries
+├── ...
+└── node
+    ├── DEPS
+    ├── ...
+    ├── build/
+    ├── node/
+    └── tools/
+```
+
+The `node_gn` is a workspace directory, which only contains configurations and
+caches of the `gclient` tool from `depot_tools`, and the repository is checked
+out at `node_gn/node`.
+
+The `node_gn/node` directory is not the repository of Node.js, it is actually
+node-ci, the repository used by V8 for testing integration with Node.js. The
+source code of Node.js is checked out at `node_gn/node/node`.
+
+### 3. Build
+
+GN only supports [`ninja`](https://ninja-build.org) for building, so to build
+Node.js with GN we have to generate `ninja` build files first and then call
+`ninja` to do the building.
+
+The `node-ci` repository provides a script for calling GN:
+
+```bash
+./tools/gn-gen.py out/Release
+```
+
+which writes `ninja` build files into the `out/Release` directory.
+
+And then you can execute `ninja`:
+
+```bash
+ninja -C out/Release node
+```
+
+## Status of the GN build
+
+Currently the GN build of Node.js is not fully functioning. It builds for macOS
+and Linux, but we are still fixing a few issues to make the Windows build work.
+And not all tests are passing using the GN build.
+
+There are also efforts on [making GN build work without using
+`depot_tools`](https://github.com/photoionization/node_with_gn), which uses a
+[forked version of GN](https://github.com/yue/build-gn).