provide bad rating contrast example in readme

[bnomei]

adding another example with a set value for contrast wich results in a bad rating could help to underline why this plugin is useful.

[nilshoerrmann]

The idea was to display the field as invalid when the contrast is too low but I wasn't able to emit the status correctly. The docs about the core components are still very sparse so I'm certainly missing the obvious. As soon as this is handled currectly additional screenshots will make more sense.