You define the plug-in with the shortcode list-github-repositories. Don’t forget to define the user whose plug-ins you want to display, else you’ll get mine.
Afterwards you get a nice list with your repositories.
Similarly you can use a shortcode like [show-github-repository user="slothsoft" repository="wordpress-github-repositories"] to show a single repository’s HTML. GitHub parses the HTML itself, so you don’t need to fear unsupported markup.