Add require call to usage example

Author: fiznool

README.md

@@ -78,6 +78,8 @@ This can be useful if you'd like to reuse strategy objects but under a different
 Most passport strategies that use OAuth 2.0 should work without any additional configuration. Some strategies, however require custom OAuth configuration, or do not expose an oauth2 adapter for internal use. In these cases, a callback can be specified by calling the `use` function with an extra `options` parameter:
 
 ```js
+const { OAuth2 } = require('oauth');
+
 refresh.use(strategy, {
   setRefreshOAuth2() {
     return new OAuth2(/* custom oauth config */);