added Json response support

guide/src/qs_7.md

@@ -13,3 +13,29 @@ Following encodings are supported:
  If request headers contains `Content-Encoding` header, request payload get decompressed
  according to header value. Multiple codecs are not supported, i.e: `Content-Encoding: br, gzip`.
  
+## JSON Response
+
+The `Json` type allows you to respond with well-formed JSON data: simply return a value of 
+type Json<T> where T is the type of a structure to serialize into *JSON*. The 
+type `T` must implement the `Serialize` trait from *serde*.
+
+```rust
+extern crate actix_web;
+#[macro_use] extern crate serde_derive;
+use actix_web::*;
+
+#[derive(Serialize)]
+struct MyObj {
+    name: String,
+}
+
+fn index(req: HttpRequest) -> Result<Json<MyObj>> {
+    Ok(Json(MyObj{name: req.match_info().query("name")?}))
+}
+
+fn main() {
+    Application::default("/")
+        .resource(r"/a/{name}", |r| r.get(index))
+        .finish();
+}
+```