Add an Explain step after Initial Commands

Following the pattern: "prepare", "do", "explain"; we want to make the guide accessible to people without coach intervention (where possible). This style of instruction always starts off telling the individual what we will be doing, then tells how to do it, then tells how to check if our command worked correctly. This makes mistakes such as creating a `railsgirls` directory inside of an already existing `railsgirls` project much more difficult, or running `rails server` in the wrong directory.

Another super common mistake is to forget to stop/start the server before typing into it. Let's be a little more declarative about what the students are doing, and empower them to solve their own problems.