Improve documentation for google driver

Signed-off-by: David Gageot <david@gageot.net>

docs/drivers/gce.md

@@ -10,19 +10,40 @@ parent="smn_machine_drivers"
 
 # Google Compute Engine
 
-Create machines on [Google Compute Engine](https://cloud.google.com/compute/). You will need a Google account and project name.
+Create machines on [Google Compute Engine](https://cloud.google.com/compute/).
+You will need a Google account and project name.
 See https://cloud.google.com/compute/docs/projects for details on projects.
 
-The Google driver uses [Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials).
+### Credentials
-So if `docker-machine` is started from a GCE host, authentication will happen automatically.
-Otherwise, you have to run `gcloud auth login` once before using `docker-machine`.
 
-Options:
+The Google driver uses [Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials)
+to get authorization credentials for use in calling Google APIs.
 
+So if `docker-machine` is used from a GCE host, authentication will happen automatically
+via the built-in service account.
+Otherwise, [install gcloud](https://cloud.google.com/sdk/) and get
+through the oauth2 process with `gcloud auth login`.
+
+### Example
+
+To create a machine instance, specify `--driver google`, the project and the machine name.
+
+```
+$ gcloud auth login
+$ docker-machine create --driver google --google-project PROJECT_ID vm01
+$ docker-machine create --driver google \
+  --google-project PROJECT_ID \
+  --google-zone us-central1-a \
+  --google-machine-type f1-micro \
+  vm02
+```
+
+### Options
+
+- `--google-project`: **required** The name of your project to use when launching the instance.
  - `--google-zone`: The zone to launch the instance.
  - `--google-machine-type`: The type of instance.
  - `--google-username`: The username to use for the instance.
- - `--google-project`: **required** The name of your project to use when launching the instance.
  - `--google-scopes`: The scopes for OAuth 2.0 to Access Google APIs. See [Google Compute Engine Doc](https://cloud.google.com/storage/docs/authentication).
  - `--google-disk-size`: The disk size of instance.
  - `--google-disk-type`: The disk type of instance.
@@ -30,16 +51,16 @@ Options:
  - `--google-preemptible`: Instance preemptibility.
  - `--google-tags`: Instance tags (comma-separated).
 
-The GCE driver will use the `ubuntu-1404-trusty-v20150909a` instance type unless otherwise specified.
+The driver uses the `ubuntu-1404-trusty-v20150909a` disk image.
 
 Environment variables and default values:
 
 | CLI option                | Environment variable  | Default                              |
 |---------------------------|-----------------------|--------------------------------------|
+| **`--google-project`**    | `GOOGLE_PROJECT`      | -                                    |
 | `--google-zone`           | `GOOGLE_ZONE`         | `us-central1-a`                      |
 | `--google-machine-type`   | `GOOGLE_MACHINE_TYPE` | `n1-standard-1`                      |
 | `--google-username`       | `GOOGLE_USERNAME`     | `docker-user`                        |
-| **`--google-project`**    | `GOOGLE_PROJECT`      | -                                    |
 | `--google-scopes`         | `GOOGLE_SCOPES`       | `devstorage.read_only,logging.write` |
 | `--google-disk-size`      | `GOOGLE_DISK_SIZE`    | `10`                                 |
 | `--google-disk-type`      | `GOOGLE_DISK_TYPE`    | `pd-standard`                        |