Suggestion: Add to Docs that Vue 2.3.0+ is Required

[bartboy011]

After spending some time trying to figure out why my modals weren't closing in a legacy app running Vue 2.2.4, I discovered that the sync modifier had a brief deprecation period from 2.0 to 2.3 and that data would only be passed one-way during this time. Updating to Vue 2.3.0 fixed this behavior!