What do you do with GitHub repositories you no longer maintain?

For whatever reason a project is started with a GitHub repository and then sometime later it's abandoned Perhaps it was an experiment that didn't work out. Perhaps you replaced it with a commercial product. Or perhaps you found a similar project to what you were doing and joined their efforts instead.

In the time your repository was alive, it attracted watchers and a few forks.

What do you do with it at that point? Is there a way to nicely indicate that repository is no longer maintained and to either check out the forks or a different project?