Update readme

Author: Guantong

README.md

@@ -2,6 +2,7 @@
 
 ![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/openai-rs/openai-api/rust.yml?style=flat-square)
 ![Crates.io](https://img.shields.io/crates/v/openai_api_rust?style=flat-square)
+![Crates.io](https://img.shields.io/crates/d/openai_api_rust?style=flat-square)
 ![GitHub](https://img.shields.io/github/license/openai-rs/openai-api?style=flat-square)
 
 A community-maintained library provides a simple and convenient way to interact with the OpenAI API.