chore: README add badge for MSRV test

Author: zonyitoo

README.md

@@ -2,6 +2,7 @@
 
 [![License](https://img.shields.io/github/license/zonyitoo/shadowsocks-rust.svg)](https://github.com/zonyitoo/shadowsocks-rust)
 [![Build & Test](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-and-test.yml/badge.svg)](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-and-test.yml)
+[![Build MSRV](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-msrv.yml/badge.svg)](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-msrv.yml)
 [![Build Releases](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-release.yml/badge.svg)](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-release.yml)
 [![Build Nightly Releases](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-nightly-release.yml/badge.svg)](https://github.com/shadowsocks/shadowsocks-rust/actions/workflows/build-nightly-release.yml)