Include documentation on adding a plugin to the app

[blefebvre]

Navigate to the phonegap shell root (assumes Bash):

$ pushd content-dev/src/main/content/jcr_root/content/phonegap/kitchen-sink/shell/_jcr_content/pge-app/app-content/phonegap

Add the plugin using the Cordova CLI:

$ cordova plugin add https://github.com/bluecats/bluecats-phonegap.git

Return to the app root, and re-install the app to AEM:

$ popd
$ mvn -PautoInstallPackage clean install