Add missing serde::{de, ser} top level comment

Author: khvzak

src/serde/de.rs

@@ -1,3 +1,5 @@
+//! Deserialize Lua values to a Rust data structure.
+
 use std::cell::RefCell;
 use std::os::raw::c_void;
 use std::rc::Rc;

src/serde/ser.rs

@@ -1,3 +1,5 @@
+//! Serialize a Rust data structure into Lua value.
+
 use serde::{ser, Serialize};
 
 use super::LuaSerdeExt;