Add message for folks on old versions of Vim

This plugin requires a relatively recent version of Vim. We should note this in the readme and provide some sort of intelligible warning for folks who try using this plugin on older versions.

Start by reading the README and identifying how the Vim plugin currently handles version compatibility. Confirm the minimum supported Vim version and define where the warning should appear; the work is done when the README documents the requirement and users on older Vim versions receive an intelligible warning.

Written by the indexing model from the issue text.