Making the Rails starter kit work out of the box

At the moment, the RoR starter kit doesn't work out of the box, because prismic.yml.sample first needs to be renamed into prismic.yml. It makes a lot of sense to make sure people change the configuration of their API endpoint, but that's not the way we documented the starter kits to work, notably [in our "Getting started" video](https://developers.prismic.io/documentation/UjBaQsuvzdIHvE4D/getting-started).

What we say in the video and documentation is that the starter kits all work out-of-the-box, but use the "Les Bonnes Choses" API, which needs changing.

To make sure people would know how to do this, I updated the README of the starter kit as well.

Let me know if I did right, or if we need to change the documentation.