update readme to include toolbar configuration options for easier reference

[tjbarker]

thanks for this gem!

I just implemented it and struggled to find toolbar options. Thought it might be helpful to have the options in the README for future referencing

[blocknotes]

Hi @tjbarker.
Thank you for you PR but I would prefer to avoid adding Quill's reference options in the gem because the purpose of
activeadmin_quill_editor is to provide just a wrapper and the right place to look for documentation about the editor should be their site only. Also consider that they could change the default toolbar buttons and I should monitor for changes to reflect those information in the README.