Add information regarding --harmony-generator support

This pull request adds a warning to the Install section of the README file, regarding the use of ES6 `yield`:

__Please Note:__ To use the `yield` functionality of rethinkdbdash, you must be running __node 0.11.9__ or higher for generator support, and must run node(1) with the `--harmony` flag. If you don't like typing this, add an alias to your shell profile:
 
 ```
 alias node='node --harmony'
 ```