[fix #3283] adding the note to docstring (#3298)

* Adding the note to the backend function

* addressing the new master change
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

torchvision/__init__.py

@@ -64,6 +64,10 @@ def set_video_backend(backend):
             The :mod:`video_reader` package includes a native C++ implementation on
             top of FFMPEG libraries, and a python API of TorchScript custom operator.
             It is generally decoding faster than :mod:`pyav`, but perhaps is less robust.
+
+    .. note::
+        Building with FFMPEG is disabled by default in the latest master. If you want to use the 'video_reader'
+        backend, please compile torchvision from source.
     """
     global _video_backend
     if backend not in ["pyav", "video_reader"]: