Add link to Dwork's seminal Differential Privacy

An essential topic in our machine learning and big data-hungry world.

information_theory/README.md

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+* :scroll: [A Mathematical Theory of Communication](http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf)
+
+* [Differential Privacy](http://www.msr-waypoint.com/pubs/64346/dwork.pdf)
+  - How do we quantify the exposure an individual faces from being
+    included in a statistical dataset?  How do we anonymize aggregated
+    data in a way that has formal guarantees?