Added Secret Key Cryptography

Hi,

Stjepan from Manning here again. This time I've added the latest Cryptography title in our catalog: Secret Key Cryptography by Frank Rubin. Thank you for considering it.

Best,

README.md

@@ -41,6 +41,7 @@ Table of Contents
    * https://www.manning.com/books/real-world-cryptography - Learn and apply cryptographic techniques.
    * https://www.manning.com/books/making-sense-of-cyber-security - A guide to the key concepts, terminology, and technologies of cybersecurity perfect for anyone planning or implementing a security strategy.
    * https://www.manning.com/books/cyber-security-career-guide - Kickstart a career in cyber security by learning how to adapt your existing technical and non-technical skills.
+   * https://www.manning.com/books/secret-key-cryptography - A book about cryptographic techniques and Secret Key methods.
 
 
 ## Documentation