Exclude documentation by default

Jekyll's default behavior is to publish all [non-excluded files](https://help.github.com/articles/files-that-start-with-an-underscore-are-missing), including `README.md` and `CONTRIBUTING.md`. 

This is fine when the documentation is public, but if Jekyll is being used to publish a closed source site, e.g., a private GitHub repository, this default behavior may potentially leak information a repository owner may somewhat righftully assume to otherwise be private.

Should we exclude `README.*` and `CONTRIBUTING.*` files by default for all private repos (allowing the user to override via the `includes` directive)?

Although the alternative is potentially embarrassing, I'm not sure how well this approach would fit within the context of "Don’t try to outsmart the users" and "Jekyll should do what I tell it to do, no more, no less". Perhaps the situation is better resolved by a warning in the docs?