Document serde dependency in JSON examples

Closes #818

examples/json_typed.rs

@@ -4,6 +4,7 @@
 //! Rust compile-time type system guaranties though it requires a little bit
 //! more code.
 
+// These require the `serde` dependency.
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Serialize, Deserialize)]

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,

src/blocking/response.rs

@@ -196,6 +196,7 @@ impl Response {
     /// # use reqwest::Error;
     /// # use serde::Deserialize;
     /// #
+    /// // This `derive` requires the `serde` dependency.
     /// #[derive(Deserialize)]
     /// struct Ip {
     ///     origin: String,