docs: add documentation on the on-behalf-of header usage (#1104)

Author: ayandyl

packages/client/USAGE.md

@@ -37,6 +37,27 @@ client.setApiKey(process.env.SENDGRID_API_KEY);
 * [USER](#user)
 * [WHITELABEL](#whitelabel)
 
+# On behalf of subusers
+
+Most API calls will accept an `on-behalf-of` header
+in order to make API calls as a given subuser:
+
+```javascript
+  // create an API key for the given subuser
+const data = {
+  name: 'subuser API key',
+  scopes: ['mail.send'],
+};
+request.body = data;
+request.method = 'POST';
+request.url = '/v3/api_keys';
+request.headers = { 'On-Behalf-Of': 'subuser username' };
+client.request(request)
+  .then(([response, body]) => {
+    console.log(response.statusCode);
+    console.log(response.body);
+  });
+```
 
 <a name="access-settings"></a>
 # ACCESS SETTINGS