Added OWASP Testing Guide (#2739)

* Added OWASP Testing Guide

* Added format type to OWASP Testing Guide

free-programming-books.md

@@ -659,6 +659,7 @@ Kerridge (PDF) (Just fill the fields with any values)
 * [How to deal with Passwords](https://github.com/MHM5000/pass)
 * [Intrusion Detection Systems with Snort](http://ptgmedia.pearsoncmg.com/images/0131407333/downloads/0131407333.pdf) (PDF)
 * [OpenSSL Cookbook](https://www.feistyduck.com/library/openssl-cookbook/)
+* [OWASP Testing Guide 4.0](https://www.owasp.org/images/1/19/OTGv4.pdf) (PDF)
 * [OWASP Top 10 for .NET Developers](http://www.troyhunt.com/2011/12/free-ebook-owasp-top-10-for-net.html)
 * [Security Engineering](http://www.cl.cam.ac.uk/~rja14/book.html)