Add documentation for use cases around ordering the sorting properties

[jscharf]

With recent discussion around how we should be leaving the order of the sorting properties to the consumer rather than sorting them in the add-on, there is an opportunity to add to the documentation of ember-frost-sort.

A few things we could cover

• When to use the sort utility that is exported from this add-on
• How to use the sort utility to order the sorting properties before passing them to the add-on