API key permissions mention in USAGE.md (#821)

Author: int-ua

USAGE.md

@@ -2,6 +2,8 @@ This documentation is based on our [OAI specification](https://github.com/sendgr
 
 # INITIALIZATION
 
+Make sure your API key has the [required permissions](https://sendgrid.com/docs/ui/account-and-settings/api-keys/).
+
 ```python
 from sendgrid import SendGridAPIClient
 import os