This seems a bit too magical to me. You assume that somewhere there will be `readme, todo, changelog` actions and you're setting up the before_filter ahead of time. It makes it harder to notice if you ever need to remove/rename/add actions.
I haven't read the rest of this pull request yet, but wouldn't it be easier to put this before_filter in the controller that uses it?
If there are several controllers, maybe create a BaseGithubController?