Also added to README.
Tested only against ckan 2.7 at this stage.

`parent-group` select box moved to separate snippet, that can be directly used
from ckanext-scheming schema. Allowable parent organisations obtained from
helper instead of previous implementation when `setup_template_variables` was
used. Thus, hierarchy_form just can be disabled

Update on the status of the 2.2 CKAN release.