Update 'README.md'

README.md

@@ -18,6 +18,7 @@ ___
 - [ZLIB](https://singlelogin.re/)
 - [Library Genesis](http://libgen.rs/)
 - [Kindle Digital Library](https://www.amazon.co.jp/hz/mycd/digital-console/contentlist/booksAll/dateDsc?pageNumber=1)
+- [Desmos](https://www.desmos.com/calculator)
 
 #### 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)*