update verilog (#2130)

* update verilog

update a free source of verilog quick reference

* substituted authoritative source for the quick reference guide

(PR has been sitting for a year)

free-programming-books.md

@@ -2300,6 +2300,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 
 ### Verilog
 
+* [Verilog Quick Reference Guide - Sutherland HDL](http://sutherland-hdl.com/pdfs/verilog_2001_ref_guide.pdf) (PDF)
 * [Verilog Tutorial](http://www.asic-world.com/verilog/veritut.html)