added the Diffie Hellman paper

cryptography/README.md

@@ -1,6 +1,7 @@
 * [A Method for Obtaining Digital Signatures and Public-Key Cryptosystems](http://people.csail.mit.edu/rivest/Rsapaper.pdf)
 * [Twenty Years of Attacks on the RSA Cryptosystem](https://crypto.stanford.edu/~dabo/papers/RSA-survey.pdf)
 * [Communication Theory of Secrecy Systems](communication-theory-of-secrecy-systems.pdf)
+* [New Directions in Cryptography](http://www-ee.stanford.edu/~hellman/publications/24.pdf)
 
 ## Related Works
 ### [A Mathematical Theory of Cryptography (1945)](http://www.cs.bell-labs.com/who/dmr/pdfs/shannoncryptshrt.pdf) - Shannon