Add a very good and comprehensive Verilog tutorial (#5264)

books/free-programming-books.md

@@ -2934,6 +2934,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### Verilog
 
+* [Verilog, Formal Verification and Verilator Beginner's Tutorial](https://zipcpu.com/tutorial/) - Daniel E. Gisselquist, Ph.D.
 * [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)