updating changelog for dup and clone semantics

activerecord/CHANGELOG

 *Rails 3.1.0 (unreleased)*
 
+* ActiveRecord::Base#dup and ActiveRecord::Base#clone semantics have changed
+to closer match normal Ruby dup and clone semantics.
+
+* Calling ActiveRecord::Base#clone will result in a shallow copy of the record,
+including copying the frozen state.  No callbacks will be called.
+
+* Calling ActiveRecord::Base#dup will duplicate the record, including calling
+after initialize hooks.  Frozen state will not be copied, and all associations
+will be cleared.  A duped record will return true for new_record?, have a nil
+id field, and is saveable.
+
 * Migrations can be defined as reversible, meaning that the migration system
 will figure out how to reverse your migration.  To use reversible migrations,
 just define the "change" method.  For example: