Update 'README.md'

README.md

@@ -85,6 +85,7 @@ ___
 - [Mathematics Genealogy Project](https://www.mathgenealogy.org/index.php)
 - [Tonnetz](https://en.wikipedia.org/wiki/Tonnetz)
 - [Counting From Infinity](https://www.youtube.com/watch?v=3JklVkYq3NM)
+- [Harmonic number](https://en.wikipedia.org/wiki/Harmonic_number)
 
 ## [Mathematics and Theoretical Computer Science](mathematics.md)
 - [OEIS: The Online Encyclopedia of Integer Sequences](https://oeis.org/)