Mention the convenience property `isInvalid`

[packages/ember-validations/lib/mixin.js#L5](https://github.com/dockyard/ember-validations/blob/2bd028670a72c962fb7ababd9ddd0125dba4df38/packages/ember-validations/lib/mixin.js#L5) has made available a `isInvalid` which is the opposite of `isValid` (duh).

It's worth mentioning in the `README.md` that it exists and can be used.