Update 'backlog.md'

backlog.md

@@ -71,6 +71,7 @@
 - [Gershgorin circle theorem](https://en.wikipedia.org/wiki/Gershgorin_circle_theorem)
 - [Science as a Vocation](https://en.wikipedia.org/wiki/Science_as_a_Vocation)
 - [Aurora](https://en.wikipedia.org/wiki/Aurora)
+- [Universal Properties and Compositionality](https://erischel.com/universal-properties-and-compositionality/)
 
 ## Reference Book Backlog
 - Bartlett's Familiar Quotations