Add a note about React/React-Native versioning

Summary:
I'm adding a readme inside the ./Libraries/Renderer folder
to explain how React and React Native are aligned.

Changelog:
[Internal] [Changed] - Add a note about React/React-Native versioning

Reviewed By: cipolleschi

Differential Revision: D36445819

fbshipit-source-id: cf1b071b9996bcc8222deab2e9458f014766f2a9

Libraries/Renderer/README.md

@@ -0,0 +1,22 @@
+# React & React Native Versions
+
+This page describes how React and React Native versions interact each other.
+The version alignment between the two frameworks relies on two syncronization points:
+
+1. The versions in the `package.json` of the new app template. For example [for React Native 0.68.1](https://github.com/facebook/react-native/blob/0.68-stable/template/package.json#L12-L15) the versions are aligned as follows:
+
+```
+  "dependencies": {
+    "react": "17.0.2",
+    "react-native": "0.68.1"
+  },
+```
+
+1. The React renderers **shipped** with React Native inside this folder, the [./Libraries/Renderer](https://github.com/facebook/react-native/tree/main/Libraries/Renderer) folder, of React Native.
+
+This practically means that you **can't bump** the version of React in your `package.json` to a later version,
+as you will still be using the older renderer from the folder mentioned above. Bumping the react version in your `package.json` will lead to unexpected behaviors.
+
+For the sake of React 18, the first version of React Native compatible with React 18 is **0.69.0**. Users on React Native 0.68.0 and previous versions won't be able to use React 18.
+
+If you use the `react-native upgrade` command or the React Native Upgrade Helper, you'll bump to the correct React version once you upgrade React Native.