docs(middleware-token): add internal package note

Co-authored-by: AllanZhengYP <[email protected]>

Author: trivikr

packages/middleware-token/README.md

@@ -4,3 +4,9 @@
 [![NPM downloads](https://img.shields.io/npm/dm/@aws-sdk/middleware-token.svg)](https://www.npmjs.com/package/@aws-sdk/middleware-token)
 
 Middleware and Plugin for setting token authentication.
+
+> An internal package
+
+## Usage
+
+You probably shouldn't, at least directly.