Update README.md

README.md

@@ -65,6 +65,7 @@ Run `rake rdoc` to generate the API docs and see [CollectiveIdea::Acts::NestedSe
 You can pass various options to `acts_as_nested_set` macro. Configuration options are:
 
 * `parent_column`: specifies the column name to use for keeping the position integer (default: parent_id)
+* `primary_column`: specifies the column name to use as the inverse of the parent column (default: id)
 * `left_column`: column name for left boundary data (default: lft)
 * `right_column`: column name for right boundary data (default: rgt)
 * `depth_column`: column name for the depth data default (default: depth)