Alert for Heroku Deployment

Hello!

I was a student at the Orlando RailsBridge earlier this year, and a TA in Savannah this past weekend. With some students now using Rails 4, we ran into some confusion when deploying to Heroku. In Rails 4, the welcome page that’s usually displayed when visiting a new application’s home page doesn't render. Instead, visitors are shown a 404 with 'The page you were looking for doesn't exist'. 

![screen shot 2013-07-16 at 9 37 16 pm](https://f.cloud.github.com/assets/2165184/809319/0430b0e8-ee82-11e2-8268-778e1d260517.png)

My proposed change includes a brief note alerting that this warning may occur until 'Setting The Default Page' later in the curriculum.