Add Mathematics Seven Sketches in Compositionality (#3317)

* Add Mathematics Seven Sketches in Compositionalit

* Update free-programming-books.md

free-programming-books.md

@@ -482,6 +482,7 @@
 * [Power Programming with Mathematica](http://mathematica.stackexchange.com/questions/16485/are-you-interested-in-purchasing-david-wagners-power-programming-with-mathemat/22724) - David B. Wagner
 * [Probability and Statistics Cookbook](http://statistics.zone)
 * [Probability and Statistics EBook](http://wiki.stat.ucla.edu/socr/index.php/Probability_and_statistics_EBook)
+* [Seven Sketches in Compositionality: An Invitation to Applied Category Theory](https://arxiv.org/pdf/1803.05316.pdf) - Brendan Fong and David I. Spivak (PDF)
 * [Statistics Done Wrong](http://www.statisticsdonewrong.com) - Alex Reinhart
 * [Think Bayes: Bayesian Statistics Made Simple](http://www.greenteapress.com/thinkbayes/) - Allen B. Downey
 * [Think Stats: Probability and Statistics for Programmers](http://greenteapress.com/thinkstats/) - Allen B. Downey (using Python)