README warns about problems with version mismatch

If you ignore the warning about the mismatch between the VirtualBox
version and the Guest Additions warning, it can have a weird effect of
disallowing symlink creation inside Shared Folders (ie /var/apps), which
breaks the provisioning stage.

See https://www.pivotaltracker.com/story/show/62766470
Merge pull request #13 from alphagov/update-readme

README warns about problems with version mismatch