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!
huh right. To make things worse `options[:input_html]` can be nil so this will blow up. Maybe we could simplify the example itself, something like:

``` ruby
super(attribute_name, options.merge(label: false), &block)
```

wdyt?