Parker Moore, on GitHub’s blog:

Today, custom domains on GitHub Pages are gaining support for HTTPS as well, meaning over a million GitHub Pages sites will be served over HTTPS.

What’s more:

We have partnered with the certificate authority Let’s Encrypt on this project. As supporters of Let’s Encrypt’s mission to make the web more secure for everyone, we’ve officially become Silver-level sponsors of the initiative.

If your custom domain uses CNAME or ALIAS records, no action is required to go HTTPS. If (like me), you have a custom domain using A records, follow along here.