docs: Add release feed badge

Author: peaceiris

README.md

@@ -3,6 +3,7 @@
 [![GitHub release date](https://img.shields.io/github/release-date/peaceiris/actions-gh-pages.svg)](https://github.com/peaceiris/actions-gh-pages/releases)
 ![GitHub Actions status](https://github.com/peaceiris/actions-gh-pages/workflows/docker%20image%20ci/badge.svg)
 [![Docker Hub Build Status](https://img.shields.io/docker/cloud/build/peaceiris/gh-pages.svg)](https://hub.docker.com/r/peaceiris/gh-pages)
+[![Release Feed](https://img.shields.io/badge/release-feed-yellow)](https://github.com/peaceiris/actions-gh-pages/releases.atom)
 
 <img width="400" alt="GitHub Actions for deploying to GitHub Pages with Static Site Generators" src="./images/ogp.svg">