explain the column-major orientation convention

[johnnychen94]

Not sure if this is documented somewhere, but the column-major orientation seems to cause some confusion and is abused somehow.

One such example is that Flux follows the same convention from Pytorch and Tensorflow, calling it WHCN order, while almost the whole community uses permutedims(channelview(X), (2, 3, 1)) to convert the data. Yes, this is more friendly to memory layout than permutedims(..., (3, 2, 1)), but it is actually HWCN order. Even the Flux community isn't fully aware of this, e.g., JuliaML/MLDatasets.jl#34

We may also need to compare other image-processing libraries; by default, MATLAB to follow the same column-major order while numpy, on the other hand, takes row-major order.

Thanks @pauljurczak for raising this issue in #142