shorten up the first section in CONTRIBUTING.md
Prevent addition of duplicate resources in Rails 4

Rails 4's ActiveModel::Name class no longer inherits from String and
breaks the expected comparison behavior when searching for existing
resources.

Redefined `ActiveAdmin::Resource::Name`'s `#eql?` and `#hash` methods to
ensure Name objects can be used as Hash keys.
Remove IH overrrides

Now the shorthand syntax works:
```ruby
controller do
  # Redirects to index page instead of rendering udpated resource
  def update
    update!{ collection_path }
  end
end
```