readme: Clarify bootloaders and board_types (#254)

- It seems to cause confusion that the H7 bootloaders are not in this
  repo.
- And we should add a note regarding board_types.txt.

Author: julianoes

README.md

@@ -1,7 +1,17 @@
-# Bootloader for the Pixhawk board family
+# Bootloader for PX4 on STM32F1/F3/F4/F7
 
 [![GitHub Actions Status](https://github.com/PX4/PX4-Bootloader/actions/workflows/main.yaml/badge.svg)](https://github.com/PX4/PX4-Bootloader/actions/workflows/main.yaml)
 
+**Note that bootloaders for STM32H7 are in [PX4](https://github.com/PX4/PX4-Autopilot) tree.**
+
+This repository is just for legacy bootloaders on STM32F1/F3/F4/F7.
+
+## Board IDs board_types.txt
+
+This repository also serves to keep track of board IDs for PX4 in [board_types.txt](https://github.com/PX4/PX4-Bootloader/blob/main/board_types.txt).
+
+It should be kept in sync with [ArduPilot's board_types.txt](https://github.com/ArduPilot/ardupilot/blob/master/Tools/AP_Bootloader/board_types.txt) as much as possible.
+
 ## Build instructions
 
 Build all targets: