Update Readme.md with configuration option about AWS IAM profiles.

It seems that Propono is using Fog, which lets the user to set a configuration option `use_iam_profile` to true, enabling the use of the aws machine credentials and automatically renew them.

By adding this to the Readme.md, it will make our life easier. We will not need to dig into this gem's code (and Fog gem's code) to know that we can use the IAM profile of our machines.

I hope this helps :)