Update README

README.md

@@ -174,18 +174,21 @@ For error reporting to the runtime APIs the library defines the `RuntimeApiError
 This library makes it easy to create Rust executables for AWS lambda. The library defines a `lambda!()` macro. Call the `lambda!()` macro from your main method with an  implementation the `Handler` type:
 
 ```rust
-pub trait Handler<E, O> {
+pub trait Handler<E, O>: Send {
+    /// Future of return value returned by handler.
+    type Future: Future<Item=O, Error=HandlerError> + Send;
+    /// IntoFuture of return value returned by handler.
+    type IntoFuture: IntoFuture<Future=Self::Future, Item=O, Error=HandlerError> + Send;
+
     /// Run the handler.
-    fn run(
-        &mut self,
-        event: E,
-        ctx: Context
-    ) -> Result<O, HandlerError>;
+    fn run(&mut self, event: E, ctx: Context) -> Self::IntoFuture;
 }
 ```
 
 `Handler` provides a default implementation that enables you to provide a Rust closure or function pointer to the `lambda!()` macro.
 
+If your handler is synchronous, you can just return a `Result` from it; if your handler is asynchronous, you can return a `Future` from it.
+
 Optionally, you can pass your own instance of Tokio runtime to the `lambda!()` macro. See our [`with_custom_runtime.rs` example](https://github.com/awslabs/aws-lambda-rust-runtime/tree/master/lambda-runtime/examples/with_custom_runtime.rs)
 
 ## AWS event objects