docs(*): add Troubleshooting Deis docs

closes #2294

/cc @johanneswuerbach 
> sporadic issues with Docker Hub...

We've had this language in the README for a while under troubleshooting, but is it still valid? The CDN and Docker Hub automation issues seem like they are largely behind us; I don't see `docker pull` fail any more often than `apt-get upgrade` lately. I wonder if we need to continue to say this, or that we're "exploring workarounds" when really we aren't pursuing any related issues with the Docker folks that I'm aware of...