Add canRejectCookies to documentation

[jvolker]

It took me a while to figure out how to add a "reject cookies" button. Turns out it's already implemented and can be used like this:

<GdprBanner cookieName="foo" description="bar" on:analytics={initAnalytics} canRejectCookies="true" />

It would be great if this were added to the documentation in the Readme.