Better docs

src/plan.rs

@@ -1,4 +1,7 @@
-//! The `plan` crate provides functionality for creating spending plans.
+//! A domain-specific language for payment plans. Users create Plan objects that
+//! are given to an interpreter. The interpreter listens for `Witness` events,
+//! which it uses to reduce the payment plan. When the plan is reduced to a
+//! `Payment`, the payment is executed.
 
 use signature::PublicKey;
 use chrono::prelude::*;