Added documentation for ActiveRecord::Associations::Builder::Association class

activerecord/lib/active_record/associations/builder/association.rb

@@ -1,3 +1,7 @@
+# This is the parent Association class which defines certain class variables (valid_options) and
+# instance variables (model, name, scope, options, reflection) which would be common across all the associations that we known today in Rails..
+# Every association need to have the values of these variables set and they are used at multiple places
+
 module ActiveRecord::Associations::Builder
   class Association #:nodoc:
     class << self