README.md: update images.csv url and docs
README.md: update images.csv url and docs

In the new minor release, two columns are added: OriginalSize and OriginalMD5. They are aimed to provide a way to validate that the downloaded images are the expected ones.

Implemented by a request in this HN thread: https://news.ycombinator.com/item?id=12615511
openimages/dataset,openimages/dataset,openimages/dataset