[doc] state explicitly that a container with padding is required

I was following the example on https://react-slick.neostack.com/
Turns out that margin is required on the slides to render the arrows and on the bottom to render the dots. There's no mention of it in the docs.
I have to inspect the DOM to find this out, both on the documentation page and example (https://codesandbox.io/s/ppwkk5l6xx).
It'll help those who haven't used jQuery slick a lot if this is stated in the docs.