Update README.md

Author: jialei777

README.md

@@ -6,3 +6,5 @@ From [Tutotial](https://docs.microsoft.com/en-us/dotnet/machine-learning/tutoria
 
 ### Object detection example 
 From [Tutotial](https://docs.microsoft.com/en-us/dotnet/machine-learning/tutorials/object-detection-onnx) and [github](https://github.com/dotnet/machinelearning-samples/tree/main/samples/csharp/getting-started/DeepLearning_ObjectDetection_Onnx)
+- need to download Tiny YOLOv2 model from [here](https://github.com/onnx/models/tree/main/vision/object_detection_segmentation/tiny-yolov2)
+to `ObjectDetect\ObjectDetect\assets\Model` directory and rename it to `TinyYolo2_model.onnx`.