Add the import statement to examples

Summary:
As a new user it took me a while to figure out you can import these examples directly. The import statement completes the example for new users like me who have no idea these components can be imported. It is a very important piece of information and it is hard to find otherwise.

I think this should be added to all the other component examples as well.

Documentation only.
Closes #15501

Differential Revision: D5882436

Pulled By: hramos

fbshipit-source-id: 2da0fe4c7c41e2fdb6b13a945460e17e16442d62

Libraries/Components/Button.js

@@ -38,6 +38,9 @@ const invariant = require('fbjs/lib/invariant');
  * Example usage:
  *
  * ```
+ * import { Button } from 'react-native';
+ * ...
+ *
  * <Button
  *   onPress={onPressLearnMore}
  *   title="Learn More"