adds documentation for row sorting feature

[y-chiasson]

Hi, I needed to implement row sorting feature, started implementing all the code myself, then I realized that Fae already implemented that using jQuery sortable and a utilities controller for handling the model updates.. so I wrote documentation for it.

Also, I couldn't figure out why row sorting wasn't getting initialized after adding the js-sort-row class to the table... It turns out there is this code in app/assets/javascripts/fae/_tables.js#L29

if (FCH.exists('.sortable-handle') || FCH.exists('.main_content-sortable-handle')) {
  this.rowSorting();
 }

which requires the sortable-handle class to exist on a td element.. so I documented that too.

[jamesmk]

This is all generated for you if you use the rails g fae:scaffold with a position attr, but we are missing documentation on the feature. This is a good start, I'll merge this and expand on it.

Thanks for the PR @y-chiasson!