Update README.md

Author: bbernhard

README.md

@@ -125,6 +125,10 @@ The Swagger API documentation can be found [here](https://bbernhard.github.io/si
 
 In case you need more functionality, please **file a ticket** or **create a PR**.
 
+## Plugins
+
+The plugin mechanism allows to register custom endpoints (with different payloads) without forking the project. Have a look [here](https://github.com/bbernhard/signal-cli-rest-api/tree/master/plugins) for details.
+
 ## Advanced Settings
 There are a bunch of environmental variables that can be set inside the docker container in order to change some technical details. This settings are meant for developers and advanced users. Usually you do *not* need to change anything here - the default values are perfectly fine!