Documenting how to implement a DataLoader for image augmentation

[rkube]

Hi, I've added an example on how to integrate MLUtils.DataLoader with an image augmentation pipeline for a custom dataset. That's a common task for CNN-based image learning. Documentation for the individual pieces (dataloading, augmentation) is good, but I couldn't find a good example of how to integrate them into one piece.

[darsnack]

This is great, thanks for the contribution! It might make more sense to add a new top-level section for "Tutorials" instead of adding more to the first page?

Also, MLUtils.jl (especially DataLoader) is the result of work directed towards building FastAI.jl. There, DataAugmentation.jl is used over Augmentor.jl. Maybe it makes sense to favor DataAugmentation.jl in the example, since future improvements would be to that package over Augmentor.jl. cc @lorenzoh for thoughts on this too.