docs(readme): add ci status badge
<br />
<div align="center">
+ <!-- CI status -->
+ <a href="https://github.com/async-rs/async-std/actions">
+ <img src="https://github.com/async-rs/async-std/workflows/CI/badge.svg"
+ alt="CI Status" />
+ </a>
<!-- Crates version -->
<a href="https://crates.io/crates/async-std">
<img src="https://img.shields.io/crates/v/async-std.svg?style=flat-square"
