added more information to the cryptol semantics hmac directory
These are files detailing the verification of HMAC with respect to the
HMAC specification provided by Andrew Appel's HMAC verification effort.
+The files in this repository constitue a proof of equivalence between
+the [Cryptol specification of HMAC](../../HMAC.cry) and the [HMAC
+specification](HMAC_spec.v) used for the FCF proof of HMAC.
+
+This proof of equivalence is done through a [mechanized semantics of
+Cryptol](https://github.com/GaloisInc/cryptol-semantics).
+
+You can read more about the FCF proof of correctness for HMAC in
+[this paper](https://www.cs.princeton.edu/~appel/papers/verified-hmac.pdf)
+
These files don't build by themselves, but rather require the
-cryptol-semantics repository
+[cryptol-semantics repository]
(https://github.com/GaloisInc/cryptol-semantics) checked out and built
in the same directory as s2n.
