Update 'README.md'

README.md

@@ -23,6 +23,7 @@ ___
 - [SciRate](https://scirate.com/)
 - [Desmos](https://www.desmos.com/calculator)
 - [Detexify](http://detexify.kirelabs.org/classify.html)
+- [Ladner's Theorem](https://en.wikipedia.org/wiki/NP-intermediate)
 
 #### Reading Stack
 - [Marx, Engels and the Abolition of the Family](https://www.csustan.edu/sites/default/files/History/Faculty/Weikart/Marx-Engels-and-the-Abolition-of-the-Family.pdf) *(added 5/3/2023)*