Clarify status of jzarr.readthedocs.io

[joshmoore]

Please see zarr-developers#18 (comment)

cc: @dchaley

[joshmoore]

cc: @SabineEmbacher @martin-boettcher et al.

[joshmoore]

Pinging again on this issue since there is still confusion within the community after the migration to http://github.com/zarr-developers/jzarr and publication on Maven central.