Add IAM_TOKEN_URL env var note to docs/getting_started.rst

Author: smithsz

docs/getting_started.rst

@@ -100,6 +100,10 @@ Cloud Platform.
 See `IBM Cloud Identity and Access Management <https://console.bluemix.net/docs/services/Cloudant/guides/iam.html#ibm-cloud-identity-and-access-management>`_
 for more information.
 
+The production IAM token service at *https://iam.bluemix.net/oidc/token* is used
+by default. You can set an ``IAM_TOKEN_URL`` environment variable to override
+this.
+
 You can easily connect to your Cloudant account using an IAM API key:
 
 .. code-block:: python