add content to Readme to better explain role, use-case for this container

[kbsingh]

we should clearly writeup what this container is for, when it makes sense, how best to use it and where to use it. Including something around howto keep your content, and the base container updated for security and bugfix updates.

[abn]

Quite a bit of useful content is available at http://rhelblog.redhat.com/2017/03/13/introducing-the-red-hat-enterprise-linux-atomic-base-image/