Update free-programming-books.md

Added an entry for the processing programming language.

free-programming-books.md

@@ -49,6 +49,7 @@
 * [Perl](#perl)
 * [PHP](#php)
 * [PowerShell](#powershell)
+* [Processing](#processing)
 * [Prolog](#prolog)
 * [PostgreSQL](#postgresql)
 * [Python](#python)
@@ -557,6 +558,10 @@
 * [Mastering PowerShell](http://powershell.com/cs/blogs/ebook/)
 
 
+###Processing
+* [The Nature of Code: Simulating Natural Systems with Processing](http://natureofcode.com/book/)
+
+
 ###Prolog
 
 * [Adventure in Prolog](http://www.amzi.com/AdventureInProlog/advfrtop.htm)