add extra comments to common mod files

src/common/from_json.rs

@@ -1,3 +1,5 @@
+//! Infrastructure trait and related functions for deserializing data from Twitter.
+
 use rustc_serialize::json;
 use error;
 use error::Error::{InvalidResponse, MissingValue};

src/common/mod.rs

@@ -1,3 +1,9 @@
+//! Set of structs and methods that act as a sort of internal prelude.
+//!
+//! The elements available in this module and its children are fairly basic building blocks that
+//! the other modules all glob-import to make available as a common language. A lot of
+//! infrastructure code goes in here.
+
 use std::borrow::Cow;
 use std::collections::HashMap;
 

src/common/response.rs

@@ -1,3 +1,6 @@
+//! Infrastructure types related to packaging rate-limit information alongside responses from
+//! Twitter.
+
 use std::vec;
 use std::iter::FromIterator;
 use std::io::Read;