First part of a user guide for GCE in jclouds

Use a numbered list here? And some text changes:
```
1. You can ask a user for consent to perform operations in their name
2. You can create a service account and authenticate using its private key
3. Unless configured otherwise, programs running on a GCE instance can perform operations as the project's [default service account](https://developers.google.com/compute/docs/authentication).
```