Update compatibility with rating menu in the readme

[bidoubiwa]

Rating Menu is now compatible with Meilisearch.

• field can only be integer, (not float)
• field only contains one value

  instantsearch.widgets.ratingMenu({
    container: "#rating-menu",
    attribute: "rating"
  }),

https://www.algolia.com/doc/api-reference/widgets/rating-menu/js/