Added A New Book Category and A Book

Added a new book category "Competitive Programming". And added to it a book "Competitive Programming" by Steven Halim, Freely downloadable from his page on the National University of Singapore.

free-programming-books.md

@@ -76,6 +76,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
     * [Algorithms & Datastructures](#algorithms--data-structures)
     * [Cellular Automata](#cellular-automata)
     * [Cloud Computing](#cloud-computing)
+    * [Competitive Programming](#competitive-programming)
     * [Compiler Design](#compiler-design)
     * [Database](#database)
     * [Datamining](#datamining)
@@ -289,6 +290,10 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [OpenStack Operations Guide](http://docs.openstack.org/ops/)
 
 
+#### Competitive Programming
+* [Competitive Programming](http://www.comp.nus.edu.sg/~stevenha/myteaching/competitive_programming/cp1.pdf) (PDF)
+
+
 #### Compiler Design
 * [Basics of Compiler Design (Anniversary Edition)](http://www.diku.dk/~torbenm/Basics/) - Torben Mogensen
 * [Compilers and Compiler Generators: An Introduction with C++](http://www.scifac.ru.ac.za/compilers/) - P. D. Terry