Enhance README with KV secrets information

Updated plugin description to include KV secrets engine.

Author: froque

README.md

@@ -6,7 +6,7 @@
 
 <!-- Plugin description -->
 
-This plugin provides database credentials using [Vault dynamic secrets](https://www.vaultproject.io/docs/secrets/databases). 
+This plugin provides database credentials using [Vault](https://developer.hashicorp.com/vault) [Database secrets](https://www.vaultproject.io/docs/secrets/databases) and [KV secrets](https://developer.hashicorp.com/vault/docs/secrets/kv) engines.
 
 Vault login is not handled by this plugin.