'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 version: int
:param version: Version of the document being indexed. If a file is
being reindexed, the version should be higher than the existing one
in CloudSearch.
:type fields: dict
:param fields: A dictionary of key-value pairs to be uploaded .
:type lang: string
:param lang: The language code the data is in. Only \'en\' is currently
supported'

def add(self, _id, version, fields, lang='en'):

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