Update README.md

Author: evansims

README.md

@@ -187,6 +187,7 @@ $auth0 = new Auth0([
     'domain' => '{{YOUR_TENANT}}.auth0.com',
     'clientId' => '{{YOUR_APPLICATION_CLIENT_ID}}',
     'clientSecret' => '{{YOUR_APPLICATION_CLIENT_SECRET}}',
+    `cookieSecret` => 'A long, random string of your choosing.'
 ]);
 ```