Added Vagrant configuration cookbook

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | n/a

A new cookbook article was added with an example Vagrant configuration that can be used with the Symfony Standard Edition.
I wouldn't want the reader to have questions this early in the document.  If we say something like "You need to have VirtualBox", I don't want them to ask "OK... sooo... how do I do that?" and not have some indication of what they should be doing.  The current text is straightforward and to the point.