CNAME file should be made in the public directory

According to GitHub docs

> **If you are working with a user pages repository, this should be done in the master branch.** If you're working with project pages, then it should go in the gh-pages branch of the project repository.

The current Octopress docs recommend placing the `CNAME` file in `source/`, but that doesn't achieve the above on deploy. The `CNAME` file needs to go into `public/` [so it will get copied to deploy/](https://github.com/imathis/octopress/blob/master/Rakefile#L261)