Add missing documentation for ActionText

[r-sierra]

Hi there!, first of all I wanna thank everyone for this great gem :rocket: .

I was trying to use this gem with an input defined with has_rich_text but could not find any reference on how to do it. Found this issue #1638, and near the end of the discussion found some comments, with suggestions on how to use it. So I decided to add this missing documentation for future references.

[MatheusRich]

@nashby is there anything actionable here?

[calebwoods]

Just discovered this exists as well. Would be great to merge this in so the documentation is up to date on what is supported out of the box

[nashby]

@r-sierra thanks! I've pushed https://github.com/heartcombo/simple_form/commit/62149be7f093ed67cdb23a5a7a4eaaee6946b9b8 instead of your change since we don't map rich_text_area to any DB type currently. Potentially we can check if model responds to rich_text methods but we're not doing it now.