Add options usage notes to README. (#316)

https://github.com/ruby-grape/grape-entity/blob/master/lib/grape_entity/entity.rb#L47

Author: drn

README.md

@@ -220,7 +220,8 @@ class ExampleEntity < Grape::Entity
 end
 ```
 
-You have always access to the presented instance with `object`
+You always have access to the presented instance (`object`) and the top-level
+entity options (`options`).
 
 ```ruby
 class ExampleEntity < Grape::Entity
@@ -229,7 +230,7 @@ class ExampleEntity < Grape::Entity
  private
 
  def formatted_value
- "+ X #{object.value}"
+ "+ X #{object.value} #{options[:y]}"
  end
 end
 ```