Example - async-await - require rust nightly-2019-05-09 version:

- Update README to reflect the required nightly version
- Add rust-toolchain with the specific version

examples/README.md

@@ -43,9 +43,13 @@ as well as code.
 2) Add [`features = ["async-await-preview"]`][feature] to the
 `tower-web` dependency.
 3) Use the necessary [nightly features] in the application.
-4) Import Tokio's [`await!` macro][await].
+4) Import Tokio's [`await!` macro][await]. *
 5) Define [`async`][async-handler] handlers.
 
+\* You should use nightly version prior to `nightly-2019-05-09`.
+As of that version the syntax change for `.await` syntax and `tokio` 0.1
+is probably not going to track nightly changes.
+
 Support for serving data over TLS is provided with the rustls feature.
 The [`rustls`](rustls) directory contains an example along with a
 [Cargo.toml](rustls/Cargo.toml) file.

examples/async-await/rust-toolchain

@@ -0,0 +1 @@
+nightly-2019-05-08