p751sidh: add README content

README.md

@@ -1,3 +1,23 @@
-```
-[:ISOGENIES:]
-```
+# `p751sidh`
+
+The `p751sidh` package provides a Go implementation of  (ephemeral)
+supersingular isogeny Diffie-Hellman, as described in [Costello-Longa-Naehrig 2016](https://eprint.iacr.org/2016/413).
+Internal functions useful for the implementation are published
+in the p751toolbox package.
+
+The implementation is intended for use on the `amd64` architecture only -- no
+generic field arithmetic implementation is provided.  Portions of the field
+arithmetic were ported from the Microsoft Research implementation.
+
+This package follows their naming convention, writing "Alice" for the party
+using 2^e-isogenies and "Bob" for the party using 3^e-isogenies.
+
+This package does NOT implement SIDH key validation, so it should only be
+used for ephemeral DH.  Each keypair should be used at most once.
+
+If you feel that SIDH may be appropriate for you, consult your
+cryptographer.
+
+Special thanks to [Craig Costello](http://www.craigcostello.com.au/), [Diego Aranha](https://sites.google.com/site/dfaranha/), and [Deirdre Connolly](https://twitter.com/durumcrustulum) for advice
+and discussion.
+