docs: add rust version icon

Signed-off-by: Zhangjian He <hezhangjian97@gmail.com>

Author: hezhangjian

README.md

@@ -3,3 +3,4 @@
 ![OpenAPI](https://img.shields.io/badge/spec-OpenAPI%203.1-blue)
 [![npm](https://img.shields.io/npm/v/apollo-openapi?label=npm&color=cb3837&logo=npm)](https://www.npmjs.com/package/apollo-openapi)
 [![PyPI version](https://img.shields.io/pypi/v/apollo-openapi.svg)](https://pypi.org/project/apollo-openapi)
+[![Rust version](https://img.shields.io/crates/v/apollo-openapi.svg)](https://crates.io/crates/apollo-openapi)