[azure-keyvault] Mentioning the ms-rest-azure requirements (#5171)

lib/services/keyVault/README.md

@@ -1,8 +1,12 @@
 # Microsoft Azure SDK for Node.js - Key Vault
 
-This project provides a Node.js package for accessing keys, secrets and certificates on Azure Key Vault. Right now it supports:
+This project provides a Node.js package for accessing keys, secrets and certificates on Azure Key Vault.
+
+It supports:
 - **Node.js version: 6.x.x or higher**
-- **REST API version: 7.0**
+- **Keyvault REST API version: 7.0**
+
+> Note: Only version 2.6.0 of `ms-rest-azure` package can be used to authenticate. Higher versions of this package are not supported.
 
 ## Features