add MSRV policy to README

this is copied verbatim from `lib.rs`

Author: rursprung

README.md

@@ -19,3 +19,7 @@ at your option.
 Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the
 work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
 additional terms or conditions.
+
+## Minimum Supported Rust Version (MSRV)
+This crate is guaranteed to compile on latest stable Rust. It *might* compile on older
+versions but that may change in any new patch release.