[README] Add a version badge (#486)

### Motivation

Add a version badge to make it easier to see the latest version.

### Modifications

Add the badge.

### Result

Easier to get the latest version.

### Test Plan

N/A

Author: czechboy0

README.md

@@ -2,6 +2,7 @@
 
 [![](https://img.shields.io/badge/sswg-sandbox-lightgrey.svg)](https://www.swift.org/sswg/)
 [![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-openapi-generator/documentation)
+[![](https://img.shields.io/github/v/release/apple/swift-openapi-generator)](https://github.com/apple/swift-openapi-generator/releases)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-generator%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-openapi-generator)
 
 Generate Swift client and server code from an OpenAPI document.