added instructions to prepare glasses 3d model in readme

Author: breathingcyborg

README.md

@@ -28,3 +28,17 @@ yarn serve
 ```
 
 Open `localhost:8080`
+
+## Preparing glasses 3d model.
+
+When using custom 3d glasses make sure the model is oriented in the right way. You can use [three js editor](https://threejs.org/editor/) or other tools like [gltfviewer](https://www.gltfviewer.com/) to check or correct orientation of glasses.
+
+![Top View](images/topview.png)
+
+![Front View](images/frontview.png)
+
+## Limitations
+
+Face Transform Module is not available in facemesh javascript. So it does not give proper depth / z coordinates. So 3d effect look weird when using perspective camera.
+
+While writing code to places objects relative to facemesh model, [facemesh model UV visualization](https://github.com/google/mediapipe/blob/master/mediapipe/modules/face_geometry/data/canonical_face_model_uv_visualization.png) can be helpful.