Document unstable_experiments.enableStandaloneFuseboxShell (#53482)

Summary:
Pull Request resolved: #53482

Changelog: [Internal]

TSIA

Reviewed By: huntie

Differential Revision: D81127995

fbshipit-source-id: cad5720b41ce409e9db1972bea3844ff0e0724ce

Author: motiz88

packages/dev-middleware/README.md

@@ -88,6 +88,16 @@ WebSocket handler for registering device connections.
 
 WebSocket handler that proxies CDP messages to/from the corresponding device.
 
+## Experimental features
+
+React Native frameworks may pass an `unstable_experiments` option to `createDevMiddleware` to configure experimental features. Note that these features might not work correctly, and they may change or be removed in the future without notice. Some of the experiment flags available are documented below.
+
+### `unstable_experiments.enableStandaloneFuseboxShell`
+
+When `true`, the debugger frontend will launch in a standalone app shell (provided by the `@react-native/debugger-shell` package) rather than in a browser window. The standalone shell provides an improved experience and will become the default in a future version of React Native.
+
+The shell is powered by a separate binary that is downloaded and cached in the background (immediately after the call to `createDevMiddleware`). If there is a problem downloading or invoking this binary for the first time, the debugger frontend will revert to launching in a browser window until the next time `createDevMiddleware` is called (typically, on the next dev server start).
+
 ## Contributing
 
 Changes to this package can be made locally and tested against the `rn-tester` app, per the [Contributing guide](https://reactnative.dev/contributing/overview#contributing-code). During development, this package is automatically run from source with no build step.