Add namespace policy example

reference: varvet#12 (comment)

README.md

@@ -351,6 +351,8 @@ end
 
 ## Manually specifying policy classes
 
+### From a given class
+
 Sometimes you might want to explicitly declare which policy to use for a given
 class, instead of letting Pundit infer it. This can be done like so:
 
@@ -362,6 +364,38 @@ class Post
 end
 ```
 
+### From the place you call `authorize`
+
+Sometimes you might want to apply namespace for your policy, especially when you
+have namespace for your controller, you can simply override `policy(record)` method
+from you controller.
+
+Here is a example for rails controllers:
+
+```ruby
+# app/controllers/api/v1/base_controller.rb
+class Api::V1::BaseController < ApplicationController
+  include Pundit
+
+  private
+
+  def policy(record)
+    "#{controller_path.classify}Policy".constantize
+  end
+end
+```
+
+And add your policy
+
+```ruby
+# app/policies/api/v1/auth/offer_policy.rb
+class Api::V1::Auth::OfferPolicy < Api::V1::BasePolicy
+  def index?
+    true
+  end
+end
+```
+
 ## Just plain old Ruby
 
 As you can see, Pundit doesn't do anything you couldn't have easily done