docs(gce): Instructions for running Deis in Google Compute Engine

This is some preliminary instructions for running Deis on Google Compute Engine. I adapted it slightly from https://gist.github.com/andyshinn/a78b617b2b16a2782655 to include a `create-gce-user-data` script which re-uses the existing `contrib/coreos/user-data` as a base and adds ephemeral storage formatting to store Docker data on a volume separate from root.

There is one change to existing `contrib/coreos/user-data` which may want a second pair of eyes. The permissions for the `write_files` section were being converted by PyYAML as octal (0755 would become 493). I changed this to be a string which seems to work just fine in CoreOS cloud-init.

I'm not a programmer and don't pretend to be one. Feel free to comment on ways I might improve (or reuse existing Deis Python libs) the Python script!
I think it's worth calling out GCE support here, even if it was community-contributed. I think GCE is something we'll want to support ourselves (and I will update its images whenever we upgrade CoreOS).