Add response json example

ref https://github.com/seanmonstar/reqwest/pull/19#issuecomment-261789621

examples/response_json.rs

@@ -0,0 +1,15 @@
+// cargo run --example response_json
+extern crate reqwest;
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Debug, Deserialize)]
+struct Response {
+    origin: String,
+}
+
+fn main() {
+    let mut res = reqwest::get("https://httpbin.org/ip").unwrap();
+    println!("JSON: {:?}", res.json::<Response>());
+}