Use GitHub Pages (Jekyll) to auto-build http://keepachangelog.com

I noticed that the page at http://keepachangelog.com was not up-to-date with the content of `README.md`.  I’m pretty sure this is because people are looking at the content, but forgetting to run the build script.  (Most similar projects on GitHub are auto-built.)

So, I converted the project to use GitHub Pages.  This way, you can just update `README.md`, and http://keepachangelog.com will be up-to-date within a few minutes.