Add missing cookie banner component README

src/govuk/components/cookie-banner/README.md

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+# Cookie banner
+
+## Installation
+
+See the [main README quick start guide](https://github.com/alphagov/govuk-frontend#quick-start) for how to install this component.
+
+## Guidance and Examples
+
+Find out when to use the cookie banner component in your service in the [GOV.UK Design System](https://design-system.service.gov.uk/components/cookie-banner).
+
+## Component options
+
+Use options to customise the appearance, content and behaviour of a component when using a macro, for example, changing the text.
+
+See [options table](https://design-system.service.gov.uk/components/cookie-banner/#options-default-cookie-banner-example) for details.