docs: add readme content for signature-v4 (#1087)

* docs: add readme content for signature-v4

* update readme wording

Author: kuhe

.changeset/witty-pugs-love.md

@@ -0,0 +1,5 @@
+---
+"@smithy/signature-v4": patch
+---
+
+add readme content for signature-v4

packages/signature-v4/README.md

@@ -2,3 +2,10 @@
 
 [![NPM version](https://img.shields.io/npm/v/@smithy/signature-v4/latest.svg)](https://www.npmjs.com/package/@smithy/signature-v4)
 [![NPM downloads](https://img.shields.io/npm/dm/@smithy/signature-v4.svg)](https://www.npmjs.com/package/@smithy/signature-v4)
+
+This package contains an implementation of the (AWS Signature Version 4)[https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html]
+authentication scheme.
+
+It is internal to Smithy-TypeScript generated clients, and not generally intended for standalone usage outside this context.
+
+For custom usage, inspect the interface of the SignatureV4 class.