Add to change log.
### SDK Features
### SDK Enhancements
+* `aws/credentials`: Add EnvProviderName constant. ([#1531](https://github.com/aws/aws-sdk-go/issues/1531))
+ * Adds the "EnvConfigCredentials" string literal as EnvProviderName constant.
+ * Fixes [#1444](https://github.com/aws/aws-sdk-go/issues/1444)
### SDK Bugs
