add readme

README.md

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+# toy Diffie-Hellman key exchange
+Only works for ~most 32 bit unsigned-integers.
+
+```
+nix run git+https://git.pyrope.net/mbk/diffie-hellman
+```
+
+Run with
+- 0 arguments to generate a public `p` and `g`
+- 2 arguments (`p` and `g`) to generate the public `g^a mod p`
+- 3 arguments (`p`, `g`, and `g^b mod p`) to generate the final shared secret
+
+`a` is provided with a passwordlike input.
+You have to make your own `a`, sorry
+