Add README

README.md

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+
+# Vigenère cipher solver
+
+Uses index of coincidence and frequency analysis to find Vigenère cipher keys
+from ciphertexts
+
+## Build
+
+```bash
+make
+```
+
+## Example usage
+
+```
+./build/solve txt/shakespeare-encrypted.txt | head -c 1024
+```
+
+## References
+
+[Vigenère cipher](https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher)
+
+[Index of Coincidence](https://en.wikipedia.org/wiki/Index_of_coincidence)
+
+[Frequency analysis](https://en.wikipedia.org/wiki/Frequency_analysis)
+