ENH: Citing source in README file
For GH users who strictly or heavily use the web-view instead of a local Git, having a direct link is handy, as it does not require downloading the PDF _if_ the user wanted to go to the source of it directly. It's an alternative that allows those interested in more uploads similar to this PDF from the same author(s).

 - [ ] closes: N/A
 - [ ] tests added / passed: N/A
 - [ ] whatsnew: Added source of PDF to README file for the cheatsheet
