Clarify correct specification of dependency on serde (#525)

Closes #494

examples/async_multiple_requests.rs

@@ -4,11 +4,11 @@ extern crate futures;
 extern crate reqwest;
 extern crate tokio;
 extern crate serde;
-#[macro_use] extern crate serde_derive;
 extern crate serde_json;
 
 use futures::Future;
 use reqwest::async::{Client, Response};
+use serde::Deserialize;
 
 #[derive(Deserialize, Debug)]
 struct Slideshow {

examples/json_typed.rs

@@ -4,9 +4,11 @@
 //! Rust compile-time type system guaranties though it requires a little bit
 //! more code.
 extern crate reqwest;
-#[macro_use] extern crate serde_derive;
+extern crate serde;
 extern crate serde_json;
 
+use serde::{Deserialize, Serialize};
+
 #[derive(Debug, Serialize, Deserialize)]
 struct Post {
     id: Option<i32>,