Add documentation about Cloud ID

Signed-off-by: Kazuma (Pakio) Arimura <k.arimura96@gmail.com>

README.md

@@ -48,7 +48,8 @@ elasticsearch_exporter --help
 
 | Argument                | Introduced in Version | Description | Default     |
 | --------                | --------------------- | ----------- | ----------- |
-| es.uri                  | 1.0.2                 | Address (host and port) of the Elasticsearch node we should connect to. This could be a local node (`localhost:9200`, for instance), or the address of a remote Elasticsearch server. When basic auth is needed, specify as: `<proto>://<user>:<password>@<host>:<port>`. E.G., `http://admin:pass@localhost:9200`. Special characters in the user credentials need to be URL-encoded. | http://localhost:9200 |
+| es.uri                  | 1.0.2                 | Address (host and port) of the Elasticsearch node we should connect to. This could be a local node (`localhost:9200`, for instance), or the address of a remote Elasticsearch server. When basic auth is needed, specify as: `<proto>://<user>:<password>@<host>:<port>`. E.G., `http://admin:pass@localhost:9200`. Special characters in the user credentials need to be URL-encoded. Cannot be used at the same time with `es.cloudid`. | |
+| es.cloudid | | Cloud ID of an Elasticsearch Service cluster. Please visit [the official document](https://www.elastic.co/guide/en/cloud/current/ec-cloud-id.html) for more information about Cloud ID. Cannot be used at the same time with `es.uri`. | |
 | es.all                  | 1.0.2                 | If true, query stats for all nodes in the cluster, rather than just the node we connect to.                             | false |
 | es.cluster_settings     | 1.1.0rc1              | If true, query stats for cluster settings. | false |
 | es.indices              | 1.0.2                 | If true, query stats for all indices in the cluster. | false |
@@ -73,6 +74,8 @@ For versions greater than `1.1.0rc1`, commandline parameters are specified with
 
 The API key used to connect can be set with the `ES_API_KEY` environment variable.
 
+We provides 2 ways to specify the address of the Elasticsearch node/cluster, `es.uri` and `es.cloudid`. When both of them are not specified, default address (`http://localhost:9200`) will be used.
+
 #### Elasticsearch 7.x security privileges
 
 Username and password can be passed either directly in the URI or through the `ES_USERNAME` and `ES_PASSWORD` environment variables.