Update RTR readme (facebook#28705)

Author: jackpope

packages/react-test-renderer/README.md

@@ -1,12 +1,26 @@
-# `react-test-renderer`
+# `react-test-renderer` (DEPRECATED)
+
+## Deprecation notice
+
+`react-test-renderer` is deprecated and no longer maintained. It will be removed in a future version. As of React 19, you will see a console warning when invoking `ReactTestRenderer.create()`.
+
+### React Testing
+
+This library creates a contrived environment and its APIs encourage introspection on React's internals, which may change without notice causing broken tests. It is instead recommended to use browser-based environments such as jsdom and standard DOM APIs for your assertions.
+
+The React team recommends [`@testing-library/react`](https://testing-library.com/docs/react-testing-library/intro) as a modern alternative that uses standard APIs, avoids internals, and [promotes best practices](https://testing-library.com/docs/guiding-principles).
+
+### React Native Testing
+
+The React team recommends @testing-library/react-native as a replacement for `react-test-renderer` for native integration tests. This React Native testing-library variant follows the same API design as described above and promotes better testing patterns.
+
+## Documentation
 
 This package provides an experimental React renderer that can be used to render React components to pure JavaScript objects, without depending on the DOM or a native mobile environment.
 
 Essentially, this package makes it easy to grab a snapshot of the "DOM tree" rendered by a React DOM or React Native component without using a browser or jsdom.
 
-Documentation:
-
-[https://reactjs.org/docs/test-renderer.html](https://reactjs.org/docs/test-renderer.html)
+Documentation: [https://reactjs.org/docs/test-renderer.html](https://reactjs.org/docs/test-renderer.html)
 
 Usage: