Add platformIO to Readme (#510) (#511)

(cherry picked from commit f0e7398)

Co-authored-by: Antonio Cuadros <49162117+Acuadros95@users.noreply.github.com>

README.md

@@ -55,6 +55,7 @@ micro-ROS also offers some other ways to crosscompile it for different platforms
 - a standalone **[micro-ROS module for NuttX RTOS](https://github.com/micro-ROS/micro_ros_nuttx_app)**: this package enables the integration of micro-ROS in any NuttX RTOS workspace.
 - a standalone **[micro-ROS module for Microsoft Azure RTOS](https://github.com/micro-ROS/micro_ros_azure_rtos_app)**: this package enables the integration of micro-ROS in a Microsoft Azure RTOS workspace.
 - a set of **[micro-ROS utils for STM32CubeMX and STM32CubeIDE](https://github.com/micro-ROS/micro_ros_stm32cubemx_utils)**: this package enables the integration of micro-ROS in STM32CubeMX and STM32CubeIDE.
+- a library builder for **[PlatformIO](https://github.com/micro-ROS/micro_ros_platformio)**: this package enables the integration of micro-ROS in PlatformIO.
 - a precompiled set of **[Arduino IDE libraries](https://github.com/micro-ROS/micro_ros_arduino)**: this package enables the integration of micro-ROS in the Arduino IDE for some hardware platforms.
 - a precompiled set of **[Raspberry Pi Pico SDK libraries](https://github.com/micro-ROS/micro_ros_raspberrypi_pico_sdk)**: this package enables the integration of micro-ROS in the Raspberry Pi Pico SDK.