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.
Yeah, this was my bad. I was testing some Docker Hub stuff and, being a relative n00b to git, didn't branch this change correctly. Closing this PR and starting another.