Added note about maintenance status to README.md.

Signed-off-by: Ralph Lange <mail@lange-ralph.de>

Author: ralph-lange

README.md

@@ -1,3 +1,6 @@
+| :warning: This repository is no longer maintained. If you are interested in taking over maintenance, please contact [Arne Nordmann](https://github.com/norro) or [Ralph Lange](https://github.com/ralph-lange). |
+|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+
 # ROS 2 System Modes
 
 [![License](https://img.shields.io/badge/License-Apache%202-blue.svg)](https://github.com/micro-ROS/system_modes/blob/master/LICENSE)
@@ -14,8 +17,6 @@ This repository explores a system modes concept that is implemented for ROS 2 in
 * [launch_system_modes](./launch_system_modes/) launch actions, events, and event handlers for system modes
 * [test_launch_system_modes](./test_launch_system_modes/) launch test for the launch_system_modes` package
 
-For further information, please contact [Arne Nordmann](https://github.com/norro) or [Ralph Lange](https://github.com/ralph-lange).
-
 ## Purpose of the Project
 
 This software is not ready for production use. It has neither been developed nor