Merge pull request #804 from chastai/add-book

Add C book "Using C on the UNIX System"

free-programming-books.md

@@ -634,7 +634,7 @@
 * [The C book](http://publications.gbdirect.co.uk/c_book/)
 * [The Craft of Text Editing or A Cookbook for an Emacs](http://www.finseth.com/craft/) - Craig A. Finseth
 * [The New C Standard - An Annotated Reference](http://www.knosof.co.uk/cbook/cbook.html)
-
+* [Using C on the UNIX System - David A. Curry](http://www.bitsinthewind.com/about-dac/publications/using-c-on-the-unix-system)
 
 ###C++
 * [C++ Annotations](http://cppannotations.sourceforge.net/)