Add some lightweight documentation.

src/signed/mod.rs

@@ -1,3 +1,14 @@
+//! This module includes a large number of signed integer types for very
+//! large integers, designed to try to match good performance with a high
+//! assurance threshold.
+//! 
+//! The types provided in this module, and the functions available for each
+//! of those types, is derived from standard bit widths for RSA, DSA, and
+//! Elliptic Curve encryption schemes. If this library does not include a
+//! function you would like for another cryptographic scheme, please reach
+//! out to the authors; in many cases, the relevant code can be automatically
+//! generated.
+//! 
 #[macro_use]
 mod add;
 #[macro_use]