Added system_modes to root index file

Author: norro

index.md

@@ -13,7 +13,9 @@ We'll update this README as we proceed. For now, you can check out our work on:
  - Micro-ROS-Agent package implementation: [micro-ROS-Agent](https://github.com/microROS/micro-ROS-Agent)
  - ROS Client Library for the C language: [rclc](https://github.com/microROS/rclc)
  - Sample code using rclc and rclcpp implementations: [micro-ROS-demos](https://github.com/microROS/micro-ROS-demos)
- - Library to support the implementation of language-specific ROS Client Libraries: [rcl](https://github.com/microROS/rcl)
+  - System modes and system hierarchy: [micro-system-modes](https://github.com/microROS/system_modes)
+ - Real-Time Operating System: [https://github.com/microROS/NuttX](https://github.com/microROS/NuttX)
+ - Micro-ROS in RTOS example applications: [apps](https://github.com/microROS/apps)
  - Real-Time Operating System: [https://github.com/microROS/NuttX](https://github.com/microROS/NuttX)
  - Micro-ROS in RTOS example applications: [apps](https://github.com/microROS/apps)
  - Reference hardware: [https://github.com/microROS/hardware](https://github.com/microROS/hardware)