add a simple README

README

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+A simple Python implementation of the HDCP key generation algorithm.
+
+This script implements a simple version of the HDCP key generation algorithm
+using the leaked HDCP master key (which is located in master-key.txt).
+
+Usage: generate_key.py [options]
+
+Options:
+  -h, --help            show this help message and exit
+  -m FILE, --master=FILE
+                        load master key from FILE
+  -k, --sink            generate a sink key rather than a source key
+  --ksv=KSV             use a specific KSV expressed in hexadecimal
+  -j, --json            output key and KSV as JSON
+
+Examples:
+
+ # Generate a sink key with KSV 0x54f0af39a8 and output the result in JSON
+ ./generate_key.py -k --ksv 54f0af39a8 -j
+
+ # Generate a source key with a random KSV and output the result in a 
+ # human-readable form
+ ./generate_key.py
+
+% vim:spelllang=en_gb:spell