Document that PRs should be opened again ome's fork

[will-moore]

For licensing reasons, contributions from the community and ome developers should go into the ome fork at https://github.com/ome/omero-zarr-pixel-buffer/.
Then, ome can migrate those changes to this repo.
This should be documented and probably added to the PR template to avoid PRs from outside of glencoesoftware being opened against this repo.

Currently we have 3 PRs from outside of glencoesoftware: #27, #28, #30.
cc @jburel @dominikl

[Rdornier]

Hello @will-moore

Ok, thanks for letting me know.
I've created a new PR against the OME repo

Rémy.