Document how to use a custom receiver app with the cast demo

#minor-release
Issue: androidx/media#452
PiperOrigin-RevId: 539915277
(cherry picked from commit 73fda2fdd4feb524b89d77c2b39d96171f62346c)

demos/cast/README.md

 # Cast demo
 
-This app demonstrates integration with Google Cast, as well as switching between
-Google Cast and local playback using ExoPlayer.
+This app demonstrates switching between Google Cast and local playback by using
+`CastPlayer` and `ExoPlayer`.
+
+## Building the demo app
 
 See the [demos README](../README.md) for instructions on how to build and run
 this demo.
+
+Test your streams by adding a `MediaItem` with URI and mime type to the
+`DemoUtil` and deploy the app on a real device for casting.
+
+## Customization with `OptionsProvider`
+
+The Cast SDK behaviour in the demo app or your own app can be customized by
+providing a custom `OptionsProvider` (see
+[`DefaultCastOptionsProvider`](https://github.com/androidx/media/blob/release/libraries/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultCastOptionsProvider.java)
+also).
+
+Replace the default options provider in the `AndroidManifest.xml` with your own:
+
+```xml
+<meta-data
+    android:name="com.google.android.gms.cast.framework.OPTIONS_PROVIDER_CLASS_NAME"
+    android:value="com.example.cast.MyOptionsProvider"/>
+```
+
+### Using a different Cast receiver app with the Media3 cast demo sender app
+
+The Media3 cast demo app is an implementation of an
+[Android Cast *sender app*](https://developers.google.com/cast/docs/android_sender)
+that uses a *default Cast receiver app* (running on the Cast device) that is
+customized to support DRM protected streams
+[by passing DRM configuration via `MediaInfo`](https://developers.google.com/cast/docs/android_sender/exoplayer).
+Hence Widevine DRM credentials can also be populated with a
+`MediaItem.DrmConfiguration.Builder` (see the samples in `DemoUtil` marked with
+`Widevine`).
+
+If you test your own streams with this demo app, keep in mind that for your
+production app you need to
+[choose your own receiver app](https://developers.google.com/cast/docs/web_receiver#choose_a_web_receiver)
+and have your own receiver app ID.
+
+If you have a receiver app already and want to quickly test whether it works
+well together with the `CastPlayer`, then you can configure the demo app to use
+your receiver:
+
+```java
+public class MyOptionsProvider implements OptionsProvider {
+  @NonNull
+  @Override
+  public CastOptions getCastOptions(Context context) {
+    return new CastOptions.Builder()
+        .setReceiverApplicationId(YOUR_RECEIVER_APP_ID)
+        // other options
+        .build();
+  }
+}
+```
+
+You can also use the plain
+[default Cast receiver app](https://developers.google.com/cast/docs/web_receiver#default_media_web_receiver)
+by using `CastMediaControlIntent.DEFAULT_MEDIA_RECEIVER_APPLICATION_ID`.
+
+#### Converting a Media3 `MediaItem` to a Cast `MediaQueueItem`
+
+This demo app uses the
+[`DefaultMediaItemConverter`](https://github.com/androidx/media/blob/release/libraries/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultMediaItemConverter.java)
+to convert a Media3 `MediaItem` to a `MediaQueueItem` of the Cast API. Apps that
+use a custom receiver app, can use a custom `MediaItemConverter` instance by
+passing it into the constructor of `CastPlayer`.
+
+### Media session and notification
+
+This Media3 cast demo app uses the media session and notification support
+provided by the Cast SDK. If your app already integrates with a `MediaSession`,
+the Cast session can be disabled to avoid duplicate notifications or sessions:
+
+```java
+public class MyOptionsProvider implements OptionsProvider {
+  @NonNull
+  @Override
+  public CastOptions getCastOptions(Context context) {
+    return new CastOptions.Builder()
+        .setCastMediaOptions(
+            new CastMediaOptions.Builder()
+                .setMediaSessionEnabled(false)
+                .setNotificationOptions(null)
+                .build())
+        // other options
+        .build();
+  }
+}
+```
+
+## Supported media formats
+
+Whether a specific stream is supported on a Cast device largely depends on the
+receiver app, the media player used by the receiver and the Cast device, rather
+then the implementation of the sender that basically only provides media URI and
+metadata.
+
+Generally, Google Cast and all Cast Web Receiver applications support the media
+facilities and types listed on
+[this page](https://developers.google.com/cast/docs/media). If you build a
+custom receiver that uses a media player different to the media player of the
+Cast receiver SDK, your app may support
+[other formats or features](https://github.com/shaka-project/shaka-player) than
+listed in the reference above.
+
+The Media3 team can't give support for building a receiver app or investigations
+regarding support for certain media formats on a cast devices. Please consult
+the Cast documentation around
+[building a receiver application](https://developers.google.com/cast/docs/web_receiver)
+for further details.