READINGS: Add Introduction to the Theory of Computation (ossu#545)

This books contains easy to understand introduction to the automata theory, which is missing in "Introduction to computing" book by David Evans. It does not require complicated math, so it can be useful to students who do not want to become theoretical computer scientists, but want do have general understanding of it's topics. Also, it almost perfectly matches The Automata Theory course we have in the curriculum.

extras/readings.md

@@ -54,6 +54,7 @@ Name | Author(s)
 Name | Author(s)
 :-- | :--:
 [Introduction to Computing: Explorations in Language, Logic, and Machines](http://www.computingbook.org/) | David Evans
+[Introduction to the Theory of Computation](https://www.amazon.com/Introduction-Theory-Computation-Michael-Sipser/dp/113318779X) | Michael Sipser
 [Introduction to Algorithms (3rd Edition)](http://www.amazon.com/Introduction-Algorithms-3rd-MIT-Press/dp/0262033844/) | Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, Clifford Stein
 [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