Add documentation on GCS repository

Author: tlrx

README.md

@@ -1,7 +1,10 @@
 Google Compute Engine Cloud Plugin for Elasticsearch
 ====================================================
 
-The GCE Cloud plugin allows to use GCE API for the unicast discovery mechanism.
+The GCE Cloud plugin allows to:
+
+* use the Google Compute Engine API for the unicast discovery mechanism
+* use the Google Cloud Storage service as a snapshot repository
 
 In order to install the plugin, run: 
 
@@ -31,6 +34,49 @@ plugin --install cloud-gce \
 ```
 
 
+Google Cloud Storage Repository
+===============================
+
+
+The gcs repository is using the [Google Cloud Storage](https://cloud.google.com/storage/) service to store snapshots. 
+The plugin uses a [service account](https://developers.google.com/accounts/docs/OAuth2ServiceAccount#creatinganaccount) authorization file 
+ to access to the GCS buckets. This way, the plugin can be used when deployed inside a Compute Engine instance or outside on a dedicated server.
+  
+To enable this fonctionnality, a service account file must be declared in the elasticsearch configuration file:
+
+```yaml
+repositories:
+    gce:
+        credentials_file: "/path/to/elasticsearch-cloud-gce-ddabc90e45.json"
+         
+```
+
+The repository can be created using the following command:
+
+```sh
+$ curl -XPUT 'http://localhost:9200/_snapshot/my_gcs_repository' -d '{
+    "type": "gcs",
+    "settings": {
+        "project_id": "my_project_id",
+        "bucket": "my_bucket_name"
+    }
+}'
+```
+
+The following settings are supported:
+
+* `project_id`: The id of the Google Cloud project. Defaults to `repositories.gcs.projet_id` or `cloud.gce.projet_id` (Mandatory)
+* `bucket`: The name of the bucket to be used for snapshots. (Mandatory)
+* `location`: The location where bucket is located. Defaults to US.
+* `base_path`: Specifies the path within bucket to repository data. Defaults to root directory.
+* `chunk_size`: Big files can be broken down into chunks during snapshotting if needed. The chunk size can be specified in bytes or by using size value notation, i.e. `1g`, `10m`, `5k`. Defaults to `100m`.
+* `compress`: When set to `true` metadata files are stored in compressed format. This setting doesn't affect index files that are already compressed by default. Defaults to `false`.
+* `buffer_size`: Size of the internal buffer used to upload snapshots to Google Cloud Storage. Defaults to `1mb`.
+* `application_name`: The name used by the plugin to connect to the Google Cloud Storage API. It can be changed to track quota. Defaults to `elasticsearch-google-cloud-storage-service`.
+
+Multiple GCS repositories can be created. Please note that all buckets must have the correct permissions configured on the Google Cloud console for the service account file.
+
+
 Google Compute Engine Virtual Machine Discovery
 ===============================