add comment to law_of_demeter_check

lib/rails_best_practices/checks/law_of_demeter_check.rb

@@ -2,6 +2,11 @@
 
 module RailsBestPractices
   module Checks
+    # Check to make sure not avoid the law of demeter.
+    # 
+    # Implementation: 
+    # 1. check all models to record belongs_to associations
+    # 2. check if calling belongs_to association's method or attribute
     class LawOfDemeterCheck < Check
 
       def interesting_nodes