Add MSRV note to README

Author: adamgreig

README.md

@@ -9,6 +9,10 @@ This project is developed and maintained by the [Cortex-M team][team].
 
 ## [Documentation](https://docs.rs/crate/cortex-m)
 
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.31 and up. It might compile with older versions but that may change in any new patch release.
+
 ## License
 
 Licensed under either of