Adding notice about Open4

Author: adamhjk

NOTICE

@@ -9,4 +9,5 @@ Contributors and Copyright holders:
  * Copyright 2008, Arjuna Christensen <aj@hjksolutions.com>
  * Copyright 2008, Bryan McLellan <btm@loftninjas.org>
  * Copyright 2008, Ezra Zygmuntowicz <ezra@engineyard.com>
- 
+
+Chef incorporates code modified from Open4 (http://www.codeforpeople.com/lib/ruby/open4/), which was written by Ara T. Howard.