Update README.md

Author: ehaupt

README.md

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 # cleanup-maildir
 
 Script for cleaning up and archiving mails in Maildir folders based on arrival date.
+This script was originally found [here](https://gist.github.com/pkern/3730543).
 
 The version of cleanup-maildir.py in this repo is ported to Python 3.8 by
 [@mhyllander](https://github.com/mhyllander/cleanup-maildir) and uses standard