Add doc for seleniumServerJar option

This options allows to let Protractor handle the selenium server.  
Unless I'm mistaken, using this option means that the webdriver_start function is not needed in the plugin and the seleniumServerJar option should therefefore be the recommended way to use it.  
The webdriver_update is still needed though.

Let me know if you want me to remove the webdriver_start or if there is a reason to keep it