docs: clarify integration tests readme

Author: mattsb42-aws

test/integration/README.rst

@@ -8,6 +8,8 @@ In order to run these integration tests successfully, these things must be confi
 #. Set environment variable ``AWS_ENCRYPTION_SDK_PYTHON_INTEGRATION_TEST_AWS_KMS_KEY_ID``
    and ``AWS_ENCRYPTION_SDK_PYTHON_INTEGRATION_TEST_AWS_KMS_KEY_ID_2`` to valid
    `AWS KMS key id`_ to use for integration tests.
+   These should be AWS KMS CMK ARNs in two different regions.
+   They will be used for integration tests.
 
 .. _automatically discoverable credential locations: http://boto3.readthedocs.io/en/latest/guide/configuration.html
 .. _AWS KMS key id: http://docs.aws.amazon.com/kms/latest/APIReference/API_Encrypt.html