Updated README to include missing setup step

I couldn't figure out why I kept getting the following error when I followed the steps to install this plugin:
```
WARN [reporter]: Can not load "notify", it is not registered!
  Perhaps you are missing some plugin?
```

After some googling, I found I needed to add the plugin to the `plugins` array in `karma.conf.js`. This pull request adds that to the example Karma configuration code in the README.