Add clarification about the downsampling factor expectation

[sbesson]

Consistently with the current implementation in

omero-zarr-pixel-buffer/src/main/java/com/glencoesoftware/omero/zarr/ZarrPixelBuffer.java

Lines 854 to 872 in 7b25b24

	@Override
	public List<List<Integer>> getResolutionDescriptions() {
	try {
	int resolutionLevels = getResolutionLevels();
	List<List<Integer>> resolutionDescriptions =
	new ArrayList<List<Integer>>();
	int sizeX = pixels.getSizeX();
	int sizeY = pixels.getSizeY();
	for (int i = 0; i < resolutionLevels; i++) {
	double scale = Math.pow(2, i);
	resolutionDescriptions.add(Arrays.asList(
	(int) (sizeX / scale), (int) (sizeY / scale)));
	}
	return resolutionDescriptions;
	} catch (Exception e) {
	// FIXME: Throw the right exception
	throw new RuntimeException(e);
	}
	}

, this updates the README to document the current expectation that resolution must be downsampled by a factor 2 alongside X and Y

Note this constraint might be up for discussion especially as recent work in #28 allows to read the scaling factor from the metadata but this would need to be captured and discussed in a dedicated issue.