README.md in nested_set-1.6.7 vs README.md in nested_set-1.6.8

- old
+ new
@@ -90,13 +90,14 @@
 
 ### References
 
 You can learn more about nested sets at:
 
-  [1](http://www.dbmsmag.com/9603d06.html)
-  [2](http://threebit.net/tutorials/nestedset/tutorial1.html)
-  [3](http://api.rubyonrails.com/classes/ActiveRecord/Acts/NestedSet/ClassMethods.html)
-  [4](http://opensource.symetrie.com/trac/better_nested_set/)
+  [1](http://en.wikipedia.org/wiki/Nested_set_model)
+  [2](http://www.ibase.ru/devinfo/DBMSTrees/9603d06.html)
+  [3](http://threebit.net/tutorials/nestedset/tutorial1.html)
+  [4](http://rdoc.info/github/rails/acts_as_nested_set/master/ActiveRecord/Acts/NestedSet/ClassMethods)
+  [5](http://agilewebdevelopment.com/plugins/betternestedset)
 
 ## How to contribute
 
 If you find what you might think is a bug:
 