A few addition:
I added the top level directory `cityscapes` since the `tools/cityscapes/convert_cityscapes_to_coco.py` script has the directory structure `gtFine_trainvaltest/gtFine` hardcoded into it which is fine but was not clear at first.

Also added a **Note** to warn people to install detectron as well, since the script uses `detectron.utils.boxes` and `detectron.utils.segm` modules which has further dependencies in the detectron lib.