docs(endpoint-cache): call out internal package

Author: trivikr

packages/endpoint-cache/README.md

@@ -2,3 +2,9 @@
 
 [![NPM version](https://img.shields.io/npm/v/@aws-sdk/endpoint-cache/latest.svg)](https://www.npmjs.com/package/@aws-sdk/endpoint-cache)
 [![NPM downloads](https://img.shields.io/npm/dm/@aws-sdk/endpoint-cache.svg)](https://www.npmjs.com/package/@aws-sdk/endpoint-cache)
+
+> An internal package
+
+## Usage
+
+You probably shouldn't, at least directly.