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.
@timglabisch Well, I haven't really come across any issues with the `rsync` type.  I suppose that line could look weird at first for someone unfamiliar with ruby, but in the next sentence of the cookbook it mentions what that developer must do to enable NFS.  This commented out part of the configuration is mainly there for convenience.  Each one of the synced folder types has their pros and cons.  Unfortunately, this cookbook cannot tell what is best for every developer and their individual environment.  With the option commented out, developers would most likely have a higher rate of success in creating the Vagrant virtual machine.

@murilolobato That's true about Windows.  I think the original problem (https://github.com/symfony/symfony-standard/pull/407#issuecomment-8529399) though relates to Linux host machines and whether they even have NFS installed, configured, and available to be used.  I don't think anyone can guarantee this, so to be more inclusive of other not-so-common developer environment configurations, the commented out line should work for a lot more people who try to use this cookbook as is, especially if they are blindly copy/pasting.