Document MSRV 1.31.0 on README

Author: eldruin

README.md

@@ -9,6 +9,11 @@ This project is developed and maintained by the [Resources team][team].
 
 ## [Documentation](https://docs.rs/mutex-trait)
 
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.31.0 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
 ## License
 
 Licensed under either of