Added Network Science book to Theoretical Computer Science section

I'm not sure exactly what section this belongs in; it seems to be influenced by several fields.  If someone else knows a better section, I'd be happy to move it.

free-programming-books.md

@@ -194,6 +194,7 @@
 * [Introduction to Computing](http://www.computingbook.org/)
 * [An Introduction to the Theory of Computation](http://www.cse.ohio-state.edu/~gurari/theory-bk/theory-bk.html)
 * [Think Complexity](http://www.greenteapress.com/compmod/) - Allen B. Downey
+* [Network Science](http://barabasilab.neu.edu/networksciencebook/index.html)
 
 ####Operating systems
 * [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/)