Add note about MSRV on README

Author: eldruin

README.md

@@ -60,6 +60,11 @@ list.
 
 [awesome-embedded-rust]: https://github.com/rust-embedded/awesome-embedded-rust#driver-crates
 
+# Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.35 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
 ## License
 
 Licensed under either of