Add example about how to read JSON file produced by VIA

[valentinitnelav]

I do this at the moment with R, but would be nice to have an example with Python.

Possibly, see:

• the function I wrote get_img_width_height_from_via_coco in https://github.com/stark-t/PAI/blob/utils_annotation/scripts/img_shape/utils_annotation.py
• https://github.com/mariolys07/VIA-to-YOLOv3/blob/main/via_to_yolo.py
• https://github.com/mjhassan/VIA-to-YOLO-annotation-converter/blob/master/via2dark.py
• https://github.com/laclouis5/globox