Update free-programming-books.md

Added a link to "Models of Computation" by John E. Savage to the
"Theoretical Computer Science" category.

free-programming-books.md

@@ -200,6 +200,7 @@
 * [Homotopy Type Theory: Univalent Foundations of Mathematics](http://homotopytypetheory.org/book/) (PDF)
 * [Introduction to Computing](http://www.computingbook.org/)
 * [Introduction to Theory of Computation](http://cg.scs.carleton.ca/~michiel/TheoryOfComputation/) (PDF) - Anil Maheshwari and Michiel Smid
+* [Models of Computation](http://cs.brown.edu/people/jes/book/) - John E. Savage
 * [Network Science](http://barabasilab.neu.edu/networksciencebook/index.html)
 * [Programming Languages: Application and Interpretation (2nd Edition)](http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/)
 * [Structure and Interpretation of Computer Programs](http://mitpress.mit.edu/sicp/)