Add link about Python 2 GZip workaround           [ci skip]
Add link about Python 2 GZip workaround           [ci skip]

Lest we or others wonder what this workaround is doing. Link back to the
PR comment for details.

TL;DR - GZip on Python 2 (and Python pre-3.5) determines the number of
bytes included via `len`, which is needed for the GZip footer. This will
be incorrect if the data is not flattened and using a single byte type.
Hence we cast to `uint8` here to satisfy this constraint. On Python 3.5+
this is not needed as it casts the data to a `memoryview` and accesses
the `nbytes` attribute to determine this.

ref: https://github.com/zarr-developers/numcodecs/pull/128#discussion_r236786466

alimanfoo/numcodecs,alimanfoo/numcodecs,zarr-developers/numcodecs