Document serde dependency in JSON examples

Closes #818

src/async_impl/response.rs

@@ -210,6 +210,7 @@ impl Response {
     /// # use reqwest::Error;
     /// # use serde::Deserialize;
     /// #
+    /// // This `derive` requires the `serde` dependency.
     /// #[derive(Deserialize)]
     /// struct Ip {
     ///     origin: String,