Update tutorial about placing sharding_filter (#487)

Summary:
See the feedback from a user: #454 (comment)

We should explicitly ask users to place `sharding_filter` as early as possible.

Pull Request resolved: #487

Reviewed By: wenleix

Differential Revision: D36812259

Pulled By: ejguan

fbshipit-source-id: 4c983f3216a80be398f85b20871e65b0e41627e0

docs/source/tutorial.rst

@@ -176,6 +176,11 @@ When we re-run, we will get:
  ...
  n_sample = 6
 
+Note:
+
+- Place ``ShardingFilter`` (``datapipe.sharding_filter``) as early as possible in the pipeline, especially before expensive
+ operations such as decoding, in order to avoid repeating these expensive operations across worker/distributed processes.
+
 
 You can find more DataPipe implementation examples for various research domains `on this page <torchexamples.html>`_.