add api doc for Either

guide/src/qs_4.md

@@ -235,6 +235,43 @@ fn main() {
 
 Both methods could be combined. (i.e Async response with streaming body)
 
+## Different return types (Either)
+
+Sometimes you need to return different types of responses. For example
+you can do error check and return error, otherwise return async response.
+Or any result that requires two different types.
+For this case [*Either*](../actix_web/enum.Either.html) type can be used.
+
+```rust
+# extern crate actix_web;
+# extern crate futures;
+# use actix_web::*;
+# use futures::future::Future;
+use futures::future::result;
+use actix_web::{Either, Error, HttpResponse, httpcodes};
+
+type RegisterResult = Either<HttpResponse, Box<Future<Item=HttpResponse, Error=Error>>>;
+
+fn index(req: HttpRequest) -> RegisterResult {
+    if true {          // <- choose variant A
+        Either::A(
+            httpcodes::HttpBadRequest.with_body("Bad data"))
+    } else {
+        Either::B(     // <- variant B
+            result(HttpResponse::Ok()
+                   .content_type("text/html")
+                   .body(format!("Hello!"))
+                   .map_err(|e| e.into())).responder())
+    }
+}
+
+fn main() {
+    Application::new()
+        .resource("/register", |r| r.f(index))
+        .finish();
+}
+```
+
 ## Tokio core handle
 
 Any actix web handler runs within properly configured