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.
Once the configuration content has been copied/pasted into a new file, it should be fairly straightforward to uncomment that option.  It is commented by default to prevent any issues for those who may not have NFS available (reference from the original discussion: https://github.com/symfony/symfony-standard/pull/407#issuecomment-8529399).

I might suggest using the `rsync` type (https://docs.vagrantup.com/v2/synced-folders/rsync.html), which was introduced sometime after this cookbook article was written, because it seems to provide much better performance since files are being read directly from the virtual machine's hard drive.  The configuration I use usually looks like this:

```
config.vm.synced_folder "..", "/vagrant", type: "rsync", rsync__exclude: [
  ".git/",
  "app/bootstrap.php.cache",
  "app/cache/",
  "app/logs/",
  "build/",
  "composer.phar",
  "vendor/",
  "web/bundles/",
  "web/uploads/",
]
```

After executing `vagrant up`, one can execute `vagrant rsync` or `vagrant rsync-auto` to update project files in the Vagrant virtual machine.