Docs Addition: GitHub Submodule Instructions

A significant ticket driver is automated build failures that occur due to a Github repository's private submodules ("Unable to clone repository!"). I included instructions/screenshots for adding the deploy key from a Hub automated build to a Github submodule in the Docker Hub > Automated Builds doc.
I'm -1 on including this here - couldn't we instead get the Hub running a build with the patch from master that fixes it, or just wait a week for 1.2.0 which will include it?