Update README.md

Updating README to account for Beaker design changes.  The major changes to the design are:

- We will be using Cordova to provide access to native device functions
- We will no longer be using an update server.  Updates will be handled using `appcache` alone (via `swapCache()`).  Multiple versions (such as live, development, etc.) will be handled by simply hosting distinct versions of the app and pointing Beaker at the desired version.