Add "A Programmer's Introduction to Mathematics" (#5553)

The website pimbook.org has a link to a "pay what you want" ebook (https://j2kun.gumroad.com/l/pim-book).

books/free-programming-books.md

@@ -544,6 +544,7 @@
 * [A First Course in Linear Algebra](http://linear.ups.edu) - Rob Beezer
 * [A Friendly Introduction to Mathematical Logic](https://milneopentextbooks.org/a-friendly-introduction-to-mathematical-logic/) - Christopher C. Leary and Lars Kristiansen
 * [A Gentle Introduction to the Art of Mathematics](https://osj1961.github.io/giam/) - Joseph E. Fields
+* [A Programmer's Introduction to Mathematics](https://pimbook.org) - Jeremy Kun
 * [Abstract Algebra: Theory and Applications](http://abstract.ups.edu) - Tom Judson
 * [Active Calculus](https://scholarworks.gvsu.edu/books/20/) - Matt Boelkins
 * [Advanced Algebra](http://www.math.stonybrook.edu/~aknapp/download/a2-alg-inside.pdf) - Anthony W. Knapp (PDF)