docs: add univention_app link in README

Author: Daenou

README.md

@@ -29,6 +29,7 @@ Name | Description
 --- | ---
 [univention.ucs_modules.univention_config_registry](./docs/univention_config_registry.md)|Manage Univention Config Registry (UCR) variables
 [univention.ucs_modules.univention_directory_manager](./docs/univention_directory_manager.md)|Manage objects via Univention Directory Manager (UDM)
+[univention.ucs_modules.univention_app](./docs/univention_app.md)|Manage univention apps on UCS
 
 ## Installing this collection