(docs) Add short crate docs to codegen.

juniper_codegen/src/lib.rs

@@ -1,3 +1,9 @@
+//! This crate supplies custom derive implementations for the
+//! [juniper](https://github.com/graphql-rust/juniper) crate.
+//!
+//! You should not depend on juniper_codegen directly.
+//! You only need the `juniper` crate.
+
 #![recursion_limit = "1024"]
 
 extern crate proc_macro;