Avoid to track untrackable fields

fields added to the schema to track changes to the document (createdAt,
updatedAt) or to the trackable fields (__updates) should not be
trackable and so must be ignored
update REDME
Avoid to track fields that doesn't exists

when you ask to track a field that is not in the model an __updates
field will be created (because you asked to track something, maybe in
the future will be useful) but it will be empty