Add note to serialization docs about zipfile format (pytorch#40288)

Summary: Pull Request resolved: pytorch#40288

Test Plan: Imported from OSS

Differential Revision: D22140324

Pulled By: jamesr66a

fbshipit-source-id: 01d7aa642ed2f4e4bdac4b7f3223bf4d7e62fd4d

Author: James Reed

docs/source/notes/serialization.rst

@@ -32,3 +32,9 @@ Then later::
 However in this case, the serialized data is bound to the specific classes
 and the exact directory structure used, so it can break in various ways when
 used in other projects, or after some serious refactors.
+
+.. note::
+    The 1.6 release of PyTorch switched ``torch.save`` to use a new
+    zipfile-based file format. ``torch.load`` still retains the ability to
+    load files in the old format. If for any reason you want ``torch.save``
+    to use the old format, pass the kwarg ``_use_new_zipfile_serialization=False``.