docs: improve stm32 embassy-usb-dfu example

examples/boot/bootloader/stm32wb-dfu/README.md

@@ -1,11 +1,37 @@
 # Bootloader for STM32
 
-The bootloader uses `embassy-boot` to interact with the flash.
+This bootloader implementation uses `embassy-boot` and `embassy-usb-dfu` to manage firmware updates and interact with the flash memory on STM32WB55 devices.
 
-# Usage
+## Prerequisites
 
-Flash the bootloader
+- Rust toolchain with `cargo` installed
+- `cargo-flash` for flashing the bootloader
+- `dfu-util` for firmware updates
+- `cargo-binutils` for binary generation
+
+## Usage
+
+### 1. Flash the Bootloader
+
+First, flash the bootloader to your device:
 
 ```
 cargo flash --features embassy-stm32/stm32wb55rg --release --chip STM32WB55RGVx
 ```
+
+### 2. Build and Flash Application
+
+Generate your application binary and flash it using DFU:
+
+```
+cargo objcopy --release -- -O binary fw.bin
+dfu-util -d c0de:cafe -w -D fw.bin
+```
+
+## Troubleshooting
+
+- Make sure your device is in DFU mode before flashing
+- Verify the USB VID:PID matches your device (c0de:cafe)
+- Check USB connections if the device is not detected
+- Make sure the transfer size option of `dfu-util` matches the bootloader configuration. By default, `dfu-util` will use the transfer size reported by the device, but you can override it with the `-t` option if needed.
+- Make sure `control_buf` size is larger than or equal to the `usb_dfu` `BLOCK_SIZE` parameter (in this example, both are set to 4096 bytes).