Readme: Add GitHub Actions status badge (#55)

Author: euanh

README.md

@@ -4,6 +4,7 @@
 [![](https://img.shields.io/github/v/release/apple/swift-container-plugin?include_prereleases)](https://github.com/apple/swift-container-plugin/releases)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-container-plugin%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-container-plugin)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-container-plugin%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/apple/swift-container-plugin)
+[![](https://github.com/apple/swift-container-plugin/actions/workflows/main.yml/badge.svg)](https://github.com/apple/swift-container-plugin/actions/workflows/main.yml/badge.svg)
 
 Publish container images using Swift Package Manager.