Don't encourage mutating args in the README

This is a really bad practice and it would be nice if popular libs like this one didn't encourage this style of programming through their README files.

Also, I :heart: simple_form, thanks!
This will not be the same. We need to pass all options and not only the `input_html`. To do what you want the code would have to be something like:

```ruby
options = options.dup
options[:input_html] = options[:input_html].merge(class: 'custom')
super(attribute_name, options, &block)
```