docs: add documentation about fromEnvSigningName

Author: trivikr

packages/token-providers/README.md

@@ -27,6 +27,15 @@ import { fromSso } from "@aws-sdk/token-providers";
 const ssoToken = await fromSso();
 ```
 
+## Env Token Provider using Signing Name
+
+```ts
+import { fromEnvSigningName } from "@aws-sdk/token-providers";
+
+// returns token from environment value stored in key computed from signing name.
+const envSigningNameToken = await fromEnvSigningName({ signingName: "signing name" });
+```
+
 ## Token Provider Chain
 
 ```ts