README to help developers install the extension and confirm that it's working

This is a README to help remind developers how to install the Ember Inspector extension and confirm that it's working by inspecting [http://try.discourse.org/](http://try.discourse.org/) and seeing:

    application
      header
      list/list
        list/topics
      modalController

and

    template=list/topics
    controller=listTopicsController
    model=<Discourse.TopicList:ember687>