Update docs README

[will-moore]

The README says you can use --output but trying seems to suggest not.
Also need to rename 'masks' to 'labels' in the README (and elsewhere?)

$ omero zarr export Image:139615 --output /Users/wmoore/Desktop/ZARR/data/
/Users/wmoore/opt/miniconda3/envs/omero zarr export: error: unrecognized arguments: --output /Users/wmoore/Desktop/ZARR/data/

[joshmoore]

Thanks, @will-moore. I'll pick these up in #25

[joshmoore]

Try: ``omero zarr --output ... ` then "export" or "masks.

As for "masks", I was trying to get across that we take omero.model.Masks from the server and turn them in Labels. Might need to make that explicit.

[will-moore]

Yep that worked:

$ omero zarr --output testoutput export Image:139615 
Using session for [email protected]:4064. Idle timeout: 10 min. Current group: read-only-1
loading plane c:0, t:0, z:0
loading plane c:1, t:0, z:0
loading plane c:2, t:0, z:0
Created testoutput/139615.zarr