Add research section to button page

Based on this comment in the backlog: alphagov/govuk-design-system-backlog#34 (comment)

src/components/button/index.md.njk

@@ -105,3 +105,9 @@ To do this, set the `data-prevent-double-click` attribute to `true`. You can do
 This feature will prevent double clicks for users that have JavaScript enabled, however you should also think about the issue server-side to protect against attacks.
 
 In the case of slow connections, aim to give the user information about what’s happening, for example, by showing a loading spinner or a modal, before using `data-prevent-double-click`.
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+## Research on this component
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+Testing on GOV.UK has shown that [the green colour of start buttons has a positive impact on click-through rates](https://github.com/alphagov/govuk-design-system-backlog/issues/34#issuecomment-699537400).
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