codemap: Add module description

src/codemap.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! This module contains utilities that work with the `CodeMap` from libsyntax / syntex_syntax.
+//! This includes extension traits and methods for looking up spans and line ranges for AST nodes.
+
 use std::rc::Rc;
 
 use syntax::codemap::{BytePos, CodeMap, FileMap, Span};