docs: add Dependabot badge

Author: peaceiris

README.md

@@ -4,6 +4,7 @@
 ![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)
+[![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=peaceiris/actions-gh-pages)](https://dependabot.com)
 
 <img width="400" alt="GitHub Actions for deploying to GitHub Pages with Static Site Generators" src="./images/ogp.svg">