Add note about image tracking requiring flags

package/Runtime/ImageTracking/README.md

@@ -4,6 +4,8 @@ Demonstrates how to attach 3D content onto a custom image marker. Start the scen
 
 You can find the image target [here](https://engine.needle.tools/samples-uploads/image-tracking/assets/needle-marker.png).
 
+Image Tracking on Android WebXR **requires a browser flag to be enabled** (chrome://flags, **WebXR Incubations**). Without this flag, image tracking is not available, and the scene needs to be placed manually. Due to this, we don't consider Image Tracking generally available.
+
 See the following links for more info:   
 - [Image Tracking Explainer](https://engine.needle.tools/docs/xr.html#image-tracking)  
 - [Everywhere Actions](https://engine.needle.tools/docs/everywhere-actions.html#image-tracking)