Update CONTRIBUTING.md (#2747)

* Update CONTRIBUTING.md

Update managing data to ask users to put data download logic to download.py instead of Makefile.

* Update CONTRIBUTING.md

Author: svekars

CONTRIBUTING.md

@@ -87,11 +87,10 @@ training data, or test data. We recommend storing this data in a
 commonly-used storage service, such as Amazon S3, and instructing your
 users to download the data at the beginning of your tutorial. 
 
-The
-[Makefile](https://github.com/pytorch/tutorials/blob/main/Makefile)
-that we use to build the tutorials contains automation that downloads
-required data files.
-
+To download your data add a function to the [download.py](https://github.com/pytorch/tutorials/blob/main/.jenkins/download_data.py)
+script. Follow the same pattern as other download functions.
+Please do not add download logic to Makefile since they will be 
+downloaded on all workers. 
 
 # Python packages used by your tutorial