Added some Documentation and renamed MONGOHQ_URL to MONGO_URL

Thanks for the great plugin. I've added some documentation and renamed the MONGOHQ_URL variable to MONGO_URL. Otherwise people think this is connected to https://mongohq.com/

What was your motivation to not allow the creation of a instance without an app? Maybe people want to share a mongodb with two apps. What do you think about using a pre-release hook to link the app to the db?