Update 'backlog.md'

backlog.md

@@ -60,6 +60,7 @@
 - [Polynomials and RE](https://blog.computationalcomplexity.org/2024/01/certifying-number-is-in-set-using.html)
 - [Holevo's Theorem](https://en.wikipedia.org/wiki/Holevo%27s_theorem)
 - [AS WE MAY THINK by VANNEVAR BUSH](http://web.mit.edu/STS.035/www/PDFs/think.pdf)
+- [A beginner's guide to forcing](https://arxiv.org/abs/0712.1320)