Update amg_example instructions (#44)

Author: cpuhrsch

amg_example/README.md

@@ -1,3 +1,5 @@
 To run this example you need to download the vit_h checkpoint and put it into a local folder named checkpoints
 
 You can find the checkpoint for vit_h here: https://dl.fbaipublicfiles.com/segment_anything/sam_vit_h_4b8939.pth
+
+To read the image you also need to install opencv-python: https://pypi.org/project/opencv-python/