Document MSRV

Closes #113

Author: yoshuawuyts

README.md

@@ -50,6 +50,14 @@ $ cargo add async-h1
 This crate uses ``#![forbid(unsafe_code)]`` to ensure everything is implemented in
 100% Safe Rust.
 
+## Minimum Supported Rust Version
+
+Given the rapidly improving nature of async Rust, only `async-h1` guarantees
+it will work on the latest stable Rust compiler. Currently `async-h1` should
+compiler on `rustc 1.40.0` and above, but we reserve the right to upgrade the
+minimum Rust version outside of major releases. If upgrading stable compiler
+versions is an issue we recommend pinning the version of `async-h1`.
+
 ## Contributing
 Want to join us? Check out our ["Contributing" guide][contributing] and take a
 look at some of these issues: