extras/readings: Add Stanford Information Retrieval (ossu#442)

Added the book "An Introduction to Information Retrieval", available on the nlp.stanford site

Author: seralf

Diff for: extras/readings.md

@@ -45,6 +45,7 @@ Name | Author(s)
 [Distributed Systems: Principles and Paradigms](https://www.amazon.com/Distributed-Systems-Principles-Andrew-Tanenbaum/dp/153028175X) | Andrew Tanenbaum
 [Distributed Systems Reading Group](http://dsrg.pdos.csail.mit.edu/papers/) | Various
 [System Design: Design large-scale systems](https://github.com/donnemartin/system-design-primer) | Various
+
 ## Theory
 
 Name | Author(s)
@@ -54,6 +55,7 @@ Name | Author(s)
 [The Algorithm Design Manual](https://www.amazon.com/gp/product/1848000693) | Steven Skiena
 [Category Theory: A Gentle Introduction](http://www.logicmatters.net/resources/pdfs/GentleIntro.pdf) | Peter Smith
 [Category Theory for Programmers: The Preface](https://bartoszmilewski.com/2014/10/28/category-theory-for-programmers-the-preface/) | Bartosz Milewski
+[An Introduction to Information Retrieval](https://nlp.stanford.edu/IR-book/pdf/irbookonlinereading.pdf) | Christopher D. Manning, Prabhakar Raghavan, Hinrich Schütze
 
 ## Applications