Add Jest entry file for external-server-runtime (facebook#25484)

Follow-up to facebook#25482.

This file is created during build, but we need an entry point for local
development, too.

Author: acdlite

packages/react-dom/unstable_server-external-runtime.js

@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+export * from 'react-dom/src/server/ReactDOMServerExternalRuntime';