Add a caution comment for installing OpenCV in Raspberry Pi

[YunjinJo]

Issue #2010
Add a caution comment in the code block to prevent a user from misconceiving.

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[d4l3k]

Here's the issues for adding prebuilt aarch64 wheels to opencv python

opencv/opencv-python#473
opencv/opencv-python#562

[d4l3k]

Looks like opencv-python has a aarch64 prebuilt library -- wonder if we should switch to that instead of contrib https://pypi.org/project/opencv-python/#files

[d4l3k]

Since we're only using cv2.VideoCapture and that's available in opencv-python does seem like we should just switch to that (or at least recommend it)

@YunjinJo have you tried just using opencv-python?

[YunjinJo]

Since we're only using cv2.VideoCapture and that's available in opencv-python does seem like we should just switch to that (or at least recommend it)

@YunjinJo have you tried just using opencv-python?

@d4l3k
I tested on my RPI (RPI 4 model B 8GB, RPI OS 64-bit) using opencv-python and it works well. I think it would be better to use opencv-python than opencv-contrib-python.