Missing `.collections()` method in `DocumentReference`

[gonzaatcroud]

The Firestore docs for the DocumentReference class include a .collections method, which lists all subcollections present in the document.

Example use case:

from mockfirestore import MockFirestore
mock_db = MockFirestore()
mock_db.collection("SOME_COLLECTION").document("SOME_DOCUMENT").collections()

Error:

AttributeError: 'DocumentReference' object has no attribute 'collections'

Link to docs: https://cloud.google.com/python/docs/reference/firestore/latest/google.cloud.firestore_v1.document.DocumentReference#google_cloud_firestore_v1_document_DocumentReference_collections

[gonzaatcroud]

Bumping this so one of the contributors can see this... @mdowds