README update

[mauro-ni]

Hi, many thanks for the gem.
Is it possible to update the Alignment (Bootstrap 4 only) example? The class d-flex is required in order to make it working.

<%= will_paginate(@things, :renderer => WillPaginate::ActionView::Bootstrap4LinkRenderer, class: 'd-flex justify-content-center') %>

Mauro

[manhcuongdtbk]

@maurosbu thanks for your comment. Without d-flex, it still works for me but the pagination stucks on the left. It's in the perfect center of the page when I add d-flex.