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

### Add setup instructions

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>

### Update vendors/ember.js 

~~Update vendors/ember.js to `Version: v1.0.0-rc.1` per todays announcement~~

Bring back the Version: `v1.0.0-pre.4-162-g1975aa3` because just swapping in  `Version: v1.0.0-rc.1` was breaking the build and I didn't catch this before.