Truncate Formatter

Do we have a wiki to stick this content into? Or maybe we can add it to the README.md?

I thought identifying how to truncate long fields may be useful for others as well.

For large character length fields that you wish to truncate and append ellipse (...), you can utilize the format_with function.

Code example:

  class MyEntity < Grape::Entity
    format_with(:truncate) { |str| str.truncate(50) }

    with_options(format_with: :truncate) do
      expose :summary
      expose :raw_content
    end
  end

This will render your content as following when the field is over 50 characters long:

{
      "summary": "adfasdf sdafjlksdaf kslafj lksdaf jlkdsfafdkasl...",
      "raw_content": "adfasdf sdafjlksdaf kslafj lksdaf jlkdsfafdkasl..."
}

Review README.md and the existing documentation around Grape::Entity and format_with. Confirm whether the example belongs in README.md or a wiki, then document the truncate formatter example and its rendered output. Done means the guidance is easy to find and accurately explains truncating long fields with an ellipsis.