Clarify xarray dependency

[rabernat]

It is not clear right now whether xarray is required dependency for rechunker and if so, what versions are required. See this Discourse thread for context: https://discourse.pangeo.io/t/issues-pip-installing-rechunker-onto-hpc-conda-environment/1034

A user reported

ImportError: cannot import name 'encode_zarr_attr_value' from 'xarray.backends.zarr' (/scratch/aeb783/penv/lib/python3.8/site-packages/xarray/backends/zarr.py)

due to an incompatible xarray version.

[TomNicholas]

Looking at the code it seems it would be pretty simple to demote xarray to be an optional dependency of rechunker?

[tomwhite]

I've opened a PR for this at #123.