Update README.md 

README.md
add Configuration block_device_mapping

Amazon EC2 Block Device Mapping Property
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-blockdev-mapping.html

 - example

```Vagrantfile
aws.block_device_mapping = [
      {
        'DeviceName' => "/dev/sda1",
        'VirtualName' => "volume_name",
        'Ebs.VolumeSize' => 100,
        'Ebs.DeleteOnTermination' => true,
        'Ebs.VolumeType' => 'standard',
      }
    ]
```