Update documentation to note requirement of 'fields' class when wrapper: false

[ajselvig]

When you pass wrapper: false to field_for, it seems like you need to contain your fields in an element with class 'fields'. Otherwise, the link_to_remove will not function. I think this is okay behavior, but it's not explicitly mentioned in the documentation.

[lest]

That would be great. Feel free to update docs and fire a pull request. Thanks!

[stsc3000]

I gave this a try since I felt it necessary to clarify the role of the wrapper somewhat in #278.

[douglasresende]

👍