Add must_use to ClientBuilder & RequestBuilder (#1011)

src/async_impl/request.rs

@@ -27,6 +27,7 @@ pub struct Request {
 }
 
 /// A builder to construct the properties of a `Request`.
+#[must_use = "RequestBuilder does nothing until you 'send' it"]
 pub struct RequestBuilder {
     client: Client,
     request: crate::Result<Request>,