[impeller] Add doc note on how to enable Impeller without the tool. (f…

…lutter#33600)

impeller/README.md

@@ -149,3 +149,26 @@ states of completion:
   there are no Impeller components that do this currently.
 
 ![Shader Compilation Pipeline](docs/shader_pipeline.png)
+
+## Try Impeller in Flutter
+
+Impeller is available under the `--enable-impeller` flag on iOS and Android. This flag can be specified to `flutter run`.
+
+If the application needs to be launched with Impeller enabled without using the Flutter tool, follow the platform specific steps below.
+
+### iOS
+
+To your `Info.plist` file, add under the top-level `<dict>` tag:
+```
+  <key>FLTEnableImpeller</key>
+  <true/>
+```
+
+### Android
+
+To your `AndroidManifest.xml` file, add under the `<application>` tag:
+```
+  <meta-data
+    android:name="io.flutter.embedding.android.EnableImpeller"
+    android:value="true" />
+```