Readme fix for rake instruction

The current Readme has 2 problems:

* It has the wrong task name.  It says "reorder:update" when it should be "reorder_children:update"
* It doesn't show what to do with bundler running (you need to do bundle exec, not just rake)

This just fixes the readme so that people don't end wasting time or contacting the mailing list over this while wondering how to progres.

Cheers,
Mohit.