patch README, warn of BUG, that rabbitmq_user is not idempotent

If my ruby skills were stronger, or if I had an hour or two to sort out how to write an effective conditional in ruby, I would offer you a real code patch.  But in the mean time, this documentation patch would have saved me an hour or two a few days ago; and another new user who posted on this issue in IRC this morning another hour.  
I suspect we are not the only ones tripped up by this issue.