Array#sum described.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54567 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -25,6 +25,8 @@ with all sufficient information, see the ChangeLog file or Redmine
     Enumerable#max and call max to an Array, your redefinition will be
     now ignored.  You should also redefine Array#max.
 
+  * Array#sum  [Feature #12217]
+
 * Dir
 
   * Dir.empty?.  [Feature #10121]