Give example for adding calculator

Believe or not but I struggled a bit with adding my own calculator to spree. As some other folks:
#1: http://grokbase.com/t/gg/spree-user/141amnc5ns/creating-spree-extension-uninitialized-constant
#2: http://stackoverflow.com/questions/5119960/how-to-properly-load-a-handmade-extension-in-spree
#3: https://www.ruby-forum.com/topic/4403156

It's all about naming convention so why not give example just in case someone forgot it (as I did!)

guides/content/developer/core/calculators.md

@@ -184,6 +184,13 @@ config.spree.calculators.shipping_methods << CustomCalculator
 config.spree.calculators.promotion_actions_create_adjustments << CustomCalculator
 ```
 
+For example if your calculator is placed in `app/models/spree/calculator/shipping/my_own_calculator.rb` you should call:
+
+```
+config = Rails.application.config
+config.spree.calculators.shipping_methods << Spree::Calculator::Shipping::MyOwnCalculator
+```
+
 ### Determining Availability
 
 By default, all shipping method calculators are available at all times. If you wish to make this dependent on something from the order, you can re-define the `available?` method inside your calculator:
@@ -239,4 +246,4 @@ plan.compute_amount(<calculable object>)
 ```
 
 `create_adjustment`, `update_adjustment` and `compute_amount` will call `compute` on the `Calculator` object. This `calculable` amount is whatever object your
-`CustomCalculator` class supports.
+`CustomCalculator` class supports.