Define root README.md for technical docs (#50333)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50333

Changelog: [internal]

This defines a placeholder for the main entrypoint for the React Native internal technical documentation.

Reviewed By: javache, mdvacca, rshest

Differential Revision: D71889755

fbshipit-source-id: 453518a0b7ec2cf433884a87446a7a622418d0b2

__docs__/README.md

@@ -0,0 +1,36 @@
+# React Native Documentation
+
+This is the internal technical documentation for React Native. It is not intended for end users of the platform.
+
+For information on our documentation strategy and structure, see [DOCS.md](./DOCS.md).
+
+## Usage
+
+This repository is not meant to be consumed directly by end users. Instead, it creates several packages that are published to the NPM registry for direct consumption by end users and frameworks.
+
+This repository uses a monorepo approach, and public packages can be found in the [`packages`](../packages/) directory (the ones that do not contain `"private": true` in their `package.json` file).
+
+The most important package is the [`react-native`](https://www.npmjs.com/package/react-native) package, located in [`packages/react-native`](../packages/react-native), which contains the public JavaScript API.
+
+This repository provides the Android and iOS versions of React Native. Versions for other platforms are maintained in their own  repositories.
+
+## Design
+
+TODO: Explain the different components of React Native at a high level.
+
+## Relationship with other systems
+
+### Part of this
+
+(TODO)
+
+### Used by this
+
+This repository has many different types of dependencies: build systems, external packages to be used during development, external packages used at runtime, etc.
+
+### Uses this
+
+The main use cases for this repository are:
+1. Developing React Native itself.
+2. Testing and releasing React Native.
+3. Synchronizing forks like `react-native-windows` and `react-native-macos`.