Move a paragraph in README to clarify where it fits in the structure.

• Before the move, the paragraph seems to apply to the --output=local section when in fact it applies to the entire --output section. This is especially confusing for the sentence "if just the path is specified as a value, buildx will use the local exporter with this path as the destination".

• After the move, it is clear that the paragraph applies to --output

I agree --progress=json is better. Thanks.

It would be awesome if buildx <bake|build> supported --progress=raw, which would output a json stream with all the structured status information, to pipe into custom progress display helpers without having to patch buildx itself.

My understanding based on Tibor's explanation is that the internal plumbing is already in place for accessing the structured status info, it's just that right now it requires linking against buildkit itself.