README: add MSRV badge

Let our README show a badge containing the MSRV.

Signed-off-by: Richard Leitner <[email protected]>

README.md

@@ -1,6 +1,7 @@
 [![crates.io version badge](https://img.shields.io/crates/v/serialport.svg)](https://crates.io/crates/serialport)
 [![Documentation](https://docs.rs/serialport/badge.svg)](https://docs.rs/serialport)
 [![GitHub workflow status](https://img.shields.io/github/actions/workflow/status/serialport/serialport-rs/ci.yaml?branch=main&logo=github)](https://github.com/serialport/serialport-rs/actions)
+[![Minimum Stable Rust Version](https://img.shields.io/badge/Rust-1.56.1-blue?logo=rust)](https://blog.rust-lang.org/2021/11/01/Rust-1.56.1.html)
 
 > **Note:** This is a fork of the original
 [serialport-rs](https://gitlab.com/susurrus/serialport-rs) project on GitLab. Please note there have