Add Drupal 8 instructions to module page

The D8 version was just added a few days ago. It took me a few minutes to figure it out so here are some basic directions since there isn't a readme.

The main difference is there is no cron run, rows are added to the index with hook_entity_update(), hook_entity_insert(), and hook_entity_delete(). This works for new nodes, but existing nodes need to be fired off manually.

1. Install Module
2. Edit settings under admin/config/system/prev-next
3. Go to your content page and under bulk actions select 'Save Content" on all nodes that you want links for.
4. Go to block settings and add the Prev/Next block
5. Profit