AP_HAL_ChibiOS: Add readme file to Here4 FC

libraries/AP_HAL_ChibiOS/hwdef/Here4FC/README.md

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+# Here4 GPS Module as Flight Controller
+
+Here4 Module is sold as a GPS module, but it can be used as a flight controller.
+
+## Features
+- STM32H757 MCU
+- ICM45686 IMU
+- MS5611 Barometer
+- RM3100 Compass
+- 1x UART
+- 2x CAN
+- 8x PWM
+- 1x RCIN
+- 1x UBLOX NEO-F9P L1L5 GNSS
+
+## How to use
+- To load the FC firmware on production Here4. User needs to first update Here4 through DroneCAN tool or Mission Planners DroneCAN config using FC firmware, which is .bin format.
+- After first update, users can connect through the serial port. Check https://docs.cubepilot.org/user-guides/here-4/here-4-manual#pinout for breakout board pinouts.
+- For enabling future updates to FC firmware, use flash bootloader through mission planner, and then user should be able to update firmware through Serial port.
+- Please note that once the firmware is updated to FC, to rollback to GPS firmware user will need to update using APJ file available here https://github.com/CubePilot/GNSSPeriph-release/releases.
+
+## Pinout and Connectors
+
+https://docs.cubepilot.org/user-guides/here-4/here-4-manual#pinout