Document whether Nydus Zran supports zstd layers

[aochagavia]

Currently, the Nydus Zran docs don't mention whether zstd layers are supported. From reading the code I got the impression that only gzip is supported at the moment (I might be wrong, though). I think it wold be worthwhile to add this information to the docs.

[imeoer]

Thanks for the mention, it's right, currently the zran implementation only support gzip layers, doc contributions is welcome. :)