Add note on Rust version requirement (#49)

On versions prior to 1.45, the compiler will error with
`procedural macros cannot be expanded to statements`.

Author: DarrenTsung

async-stream/README.md

@@ -150,6 +150,9 @@ caller.
 
 [`Stream`]: https://docs.rs/futures-core/*/futures_core/stream/trait.Stream.html
 
+## Supported Rust Versions
+`async-stream` is built against the latest stable release. The minimum supported version is 1.45 due to [function-like procedural macros in expression, pattern, and statement positions](https://blog.rust-lang.org/2020/07/16/Rust-1.45.0.html#stabilizing-function-like-procedural-macros-in-expressions-patterns-and-statements).
+
 ## License
 
 This project is licensed under the [MIT license](LICENSE).