'Add a document to be processed by the DocumentService
The document will not actually be added until :func:`commit` is called
:type _id: string
:param _id: A unique ID used to refer to this document.
:type fields: dict
:param fields: A dictionary of key-value pairs to be uploaded .'

def add(self, _id, fields):

    d = {'type': 'add', 'id': _id, 'fields': fields}
    self.documents_batch.append(d)
