README: Explain the project in more detail (#1)

Author: MarkLodato

README.md

@@ -1,2 +1,22 @@
 # signing-spec
-A specification for signing methods and formats used by Secure Systems Lab projects. 
+
+Simple, foolproof standard for signing arbitrary data.
+
+*   Why not [JOSE/JWS/JWT](https://jwt.io)? JSON-specific, too complicated, too
+    easy to mess up.
+*   Why not [PASETO](https://paseto.io)? JSON-specific, too opinionated.
+
+## What is it?
+
+*   [Signature protocol](specification.md)
+*   [Data structure](specification.md) for storing the message and signatures
+*   (pending #9) Suggested crypto primitives
+
+Out of scope (for now at least):
+
+*   Key management / PKI
+
+## Who uses it?
+
+*   [in-toto](https://in-toto.io) (pending in-toto/ITE#13)
+*   [TUF](https://theupdateframework.io) (pending)