list dependencies; say how to make

[camilleg]

On Ubuntu 22.04.3, by trial and error, to make it compile and link I had to apt install libopencv-core-dev libopencv-calib3d-dev libopencv-dnn-dev libopencv-objdetect-dev libopencv-photo-dev libopencv-stitching-dev libopencv-video-dev libopencv-dev. After fixing a few syntax errors, it built with g++ -I/usr/include/opencv4/ Main.cpp -o Main $(pkg-config --libs opencv4). There may be a cleaner way, though.