Add docs for build-types (#51852)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51852

- Add README for `yarn build-types`.
- Populate command `--help` output.

Changelog: [Internal]

Reviewed By: coado

Differential Revision: D72168391

fbshipit-source-id: d822333d6013bd2ada244e45589f6424191a69d5

scripts/build-types/README.md

@@ -0,0 +1,66 @@
+# scripts/build-types
+
+TypeScript build pipeline for React Native's JavaScript API.
+
+## Overview
+
+`yarn build-types` is a custom build pipeline for translating React Native's Flow source code to TypeScript.
+
+Specifically, it reduces the runtime JavaScript API of `react-native` into two outputs:
+
+- **Generated TypeScript types**\
+Public user types for react-native, shipped to npm\
+`packages/react-native/types_generated/`
+- **‌Public API snapshot (experimental)**\
+Snapshot file of the public API shape, used by maintainers\
+`packages/react-native/ReactNativeApi.d.ts`
+
+#### Dependencies
+
+`yarn build-types` makes use of the following dependencies, composed with other pre/post transformation steps and dependency resolution.
+
+- Flow → TypeScript conversion: [flow-api-extractor](https://www.npmjs.com/package/flow-api-translator)
+- TypeScript → (initial) API rollup: [@microsoft/api-extractor](https://api-extractor.com/)
+
+## Usage
+
+`yarn build-types` is designed to be run by maintainers with minimal arguments.
+
+> API snapshot generation is currently **experimental**, and will be folded into the default behaviour when ready.
+
+```sh
+# Build types
+yarn build-types
+
+# Build types + API snapshot (experimental)
+yarn build-types --withSnapshot [--validate]
+```
+
+#### Configuration
+
+Sparse configuration options are defined and documented in `scripts/build-types/config.js`.
+
+## About the two formats
+
+### Generated TypeScript types
+
+`types_generated/`
+
+Directory providing TypeScript user types for the `react-native` package, distributed via npm.
+
+- Gitignored.
+- Scoped to the `index.d.ts` entry point via `package.json#exports`.
+- Preserves `unstable_` and `experimental_` APIs.
+- Preserves doc comments.
+- Preserves source file names (for go to definition).
+
+### Public API snapshot (experimental)
+
+`ReactNative.d.ts`
+
+Provides a human-readable, maintainable reference of the React Native's public JavaScript API, optimized for developers and diff tooling.
+
+- Committed to the repo.
+- Strips `unstable_` and `experimental_` APIs.
+- Strips doc comments.
+- Strips source file names and some un-exported type names (WIP).

scripts/build-types/index.js

@@ -36,6 +36,9 @@ async function main() {
   Usage: node ./scripts/build-types
 
   Build generated TypeScript types for react-native.
+
+  Options:
+    --withSnapshot    [Experimental] Include API snapshot generation.
     `);
     process.exitCode = 0;
     return;