docs: add README and specify file in package.json (#28507)

Summary:
Adding a README for `react-native-codegen` since the package was published.
Also added a `files` prop in package.json so unused file won't be included in the package.

## Changelog

[Internal] [Changed] - Add README for react-native-codegen.
Pull Request resolved: https://github.com/facebook/react-native/pull/28507

Test Plan: verify js files to function correctly without including files other than `src`

Reviewed By: rickhanlonii

Differential Revision: D20836113

Pulled By: cpojer

fbshipit-source-id: e860f14760e9c1dbe121f5fb95ccf72d4ddb2af1

packages/react-native-codegen/README.md

@@ -0,0 +1,14 @@
+# react-native-codegen
+
+[![Version][version-badge]][package]
+
+## Installation
+
+```
+yarn add --dev react-native-codegen
+```
+
+*Note: We're using `yarn` to install deps. Feel free to change commands to use `npm` 3+ and `npx` if you like*
+
+[version-badge]: https://img.shields.io/npm/v/react-native-codegen?style=flat-square
+[package]: https://www.npmjs.com/package/react-native-codegen

packages/react-native-codegen/package.json

@@ -1,15 +1,19 @@
 {
-  "version": "0.0.1",
   "name": "react-native-codegen",
+  "version": "0.0.1",
   "description": "⚛️ Code generation tools for React Native",
+  "homepage": "https://github.com/facebook/react-native/tree/master/packages/react-native-codegen",
   "repository": {
     "type": "git",
     "url": "git@github.com:facebook/react-native.git"
   },
+  "license": "MIT",
+  "files": [
+    "src"
+  ],
   "dependencies": {
     "flow-parser": "^0.121.0",
     "jscodeshift": "^0.7.0",
     "nullthrows": "^1.1.1"
-  },
-  "license": "MIT"
+  }
 }