Update Heroku Addon command for new MongoHQ offerings

Hello,

I was setting up a new locomotive instance and noticed that mongohq changed their offerings on Heroku so that the documentation no longer matches.  I added the new free option to the documentation.

I have been using locomotive a lot recently and I am interested in looking for ways to contribute, not sure if this is where you maintain all the content for the doc site, but thought I would give it a go.

Robert