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!)
Merge pull request #236 from Bartuz/patch-1

Give example for adding calculator