Add nightly build badge to README

Author: Narukara

README.md

@@ -1,6 +1,8 @@
 ![banner](.images/banner-dark-theme.png#gh-dark-mode-only)
 ![banner](.images/banner-light-theme.png#gh-light-mode-only)
 
+[![Nightly](https://github.com/micro-ROS/micro_ros_espidf_component/actions/workflows/nightly.yml/badge.svg)](https://github.com/micro-ROS/micro_ros_espidf_component/actions/workflows/nightly.yml)
+
 # micro-ROS component for ESP-IDF
 
 This component has been tested in ESP-IDF v5.2, v5.3, v5.4, and v5.5 with ESP32, ESP32-S2, ESP32-S3, ESP32-C3 and ESP32-C6.