doc: Update credentials section

Add note saying not to store creds in the CfnCluster config.

Refs #345

Signed-off-by: Brian Barrett <[email protected]>

Author: bwbarrett

docs/source/configuration.rst

@@ -50,9 +50,9 @@ Attempts to validate that resources defined in parameters actually exist. ::
 
 aws
 ^^^
-This is the AWS credentials section (required).  These settings apply to all clusters.
+This is the AWS credentials/region section (required).  These settings apply to all clusters.
 
-If not defined, boto will attempt to use a) environment or b) EC2 IAM role. ::
+We highly recommend use of the environment, EC2 IAM Roles, or storing credentials using the `AWS CLI <https://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html>`_ to store credentials, rather than storing them in the CfnCluster config file. ::
 
     [aws]
     aws_access_key_id = #your_aws_access_key_id