Mention that QOI is compressable format

Unlike most of other image formats which can not be compressed further, QOI can be considerably compressed with general compression algorithms Unlike other image formats which will generate highly binary different files for partially same images, QOI generates alike files, alloving to compress variant sets with insane compression (I've got 11MB .qoi.7z out of 4x 12MB pngs)

Variant set: A CG set depicting only a single scenery from a static perspective. (they often have 50-90% of same pixels)

Please write the information about high compression rates for both single images and image sets in the readmes