Update README.md

Author: MaximilianKresse

README.md

@@ -20,6 +20,11 @@ for the requests. So you'll need an implementation of these. We recommend using
     }
 ```
 
+Additionally to request an access token you'll need an oauth2 implementation such
+as [league/oauth2-client](https://github.com/thephpleague/oauth2-client).
+Sample code for this can be found in "[examples/generate-token.php](examples/generate-token.php)".
+Note: because of oauth2 your domain MUST be available through HTTPS.
+
 ## Installation
 Add following to your composer.json:
 
@@ -61,4 +66,4 @@ component. If you want to use it with an evaluation version please use following
 
 ## License
 
-This package is open-sourced software licensed under the [MIT license](https://opensource.org/licenses/MIT).
+This package is open-sourced software licensed under the [MIT license](https://opensource.org/licenses/MIT).