ENH: Citing source in README file (#15856)

* 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).

* jorisvandenbossche's feedback

I re-read the PDF and made sure the wording reflected the content presented. I also changed the source-citing so that is more friendly for .TXT files instead of Markdown or unspecified.

* Update README.txt

* English enhancement

Improved sentence structure for English speakers.

Author: the-nose-knows

doc/cheatsheet/README.txt

@@ -2,3 +2,7 @@ The Pandas Cheat Sheet was created using Microsoft Powerpoint 2013.
 To create the PDF version, within Powerpoint, simply do a "Save As"
 and pick "PDF' as the format.
 
+This cheat sheet was inspired by the RstudioData Wrangling Cheatsheet[1], written by Irv Lustig, Princeton Consultants[2].
+
+[1]: https://www.rstudio.com/wp-content/uploads/2015/02/data-wrangling-cheatsheet.pdf
+[2]: http://www.princetonoptimization.com/