Add SharedConfigFiles update to pending change log
### SDK Features
### SDK Enhancements
+* `aws/session`: SDK should be able to load multiple custom shared config files. [#1258](https://github.com/aws/aws-sdk-go/issues/1258)
+ * This change adds a `SharedConfigFiles` field to the `session.Options` type that allows you to specify the files, and their order, the SDK will use for loading shared configuration and credentials from when the `Session` is created. Use the `NewSessionWithOptions` Session constructor to specify these options. You'll also most likely want to enable support for the shared configuration file's additional attributes by setting `session.Option`'s `SharedConfigState` to `session.SharedConfigEnabled`.
### SDK Bugs
