README: new schema_description description; renamed dataset_fields_by_ckan_id and resource_fields_by_ckan_id

Renamed schema_description.dataset_fields_by_ckan_id and resource_fields_by_ckan_id to dataset_field_iter and resource_field_iter. Original names were too similar to existing dataset_field_by_id and resource_field_by_id attributes of the same object.

Update your scripts! (sorry for the breakage)

Also we finally have documentation for the schema_description object used all over the place in our code.