Add a note regarding decorators in forms

This is a start for #2013. I'm not really sure how much detail to go into here. Is this all that's needed? or should I explain all of https://github.com/gregbell/active_admin/issues/2013#issuecomment-15317627...

Also, I think it would be nice to provide a workaround for folks that need their forms not decorated as it's not easily configurable.

Here's the hack I'm using in one project: https://gist.github.com/amiel/5320486