[readme] default dns to xip.io
default dns 1.2.3.4.xip.io [fixes #200]

Thanks to @yudai for enabling xip.io validations in 1cf12a3d5d
Enable change of dns attribute

This allows you to have a default IP.xip.io domain initially
and later change to a custom domain.

Usage:

    bosh change cf attribute dns=mycloud.com
Merge pull request #202 from cloudfoundry-community/xip

Default DNS to 1.2.3.4.xip.io & enabled dns attribute to be changed