Added "Github Submodules" section to Automated Builds doc.

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.


Signed-off-by: Hollie Teal <hollie.teal@docker.com>