readings: Add Computer Systems: A Programmer's Perspective book

This is probably the best introduction to computer systems for programmers. It contains all needed essentials of computer architecture, describes how C language works internally (e.g. how data structures are organized in memory and managed on machine language level) and even describes essentials of operating systems.

extras/readings.md

@@ -40,6 +40,7 @@ Name | Author(s)
 
 Name | Author(s)
 :-- | :--:
+[Computer Systems: A Programmer's Perspective (3rd Edition)](http://csapp.cs.cmu.edu/) | Randal E. Bryant, David R. O'Hallaron
 [Modern Operating Systems (4th Edition)](http://www.amazon.com/Modern-Operating-Systems-Andrew-Tanenbaum/dp/013359162X/) | Andrew S. Tanenbaum, Herbert Bos
 [Computer Organization and Design: The Hardware/Software Interface](https://www.amazon.com/gp/product/0124077269) | David Patternson, John Hennessy
 [Computer Networks (5th Edition)](http://www.amazon.com/Computer-Networks-5th-Andrew-Tanenbaum/dp/0132126958/) | Andrew S. Tanenbaum, David J. Wetherall