Document MSRV on README

riscv-rt/README.md

@@ -10,6 +10,11 @@ This project is developed and maintained by the [RISC-V team][team].
 
 ## [Documentation](https://docs.rs/crate/riscv-rt)
 
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.38.0 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
 ## License
 
 Copyright 2018 [RISC-V team][team]