Mention no images added to repository
Mention that images shouldn't be added to the repository as it will otherwise significantly weigh down the repository.
WIll merge this as a first step!<|||||>Merged too soon? We need a concrete destination for images that we all share.<|||||>Here it is: https://huggingface.co/datasets/huggingface/documentation-images<|||||>but it's not part of this PR that was merged. i.e. @NielsRogge's suggestion wasn't integrated into the doc.

additionally please please keep these instructions in `docs/README.md` - thank you!