Google App Engine Install instructions

The docker instructions are very simple (thank you!)

``` bash
mkdir data
cp my_config.cfg data/cayley.cfg
docker run -v $PWD/data:/data -p 64321:64321 -d quay.io/barakmich/cayley
```

Any hope of a similar small and easy Google App Engine Managed VM? It _feels_ like it should be almost as easy, especially with [Google Cloud Managed VMs](https://cloud.google.com/appengine/docs/managed-vms/) and [Google Cloud SQL](https://cloud.google.com/sql/docs/introduction). But I don't know enough about Docker and how it works with Google's Managed VMs to translate the instructions. (and if any of the SQL backing store would break it, or if a managed-VM local data store would be easiest)

Start with the Docker command in the issue and the linked Google Cloud Managed VMs and Cloud SQL documentation. Determine whether Cayley’s Docker image and a supported data store fit Managed VMs, then document a comparably small deployment path and its storage constraints.

Written by the indexing model from the issue text.