README.md: add more details to flash instructions

Signed-off-by: Martin Dummer <martin.dummer@gmx.net>

README.md

@@ -124,9 +124,10 @@ This can be achieved by editing the 'platformio.ini' file and add/change one or
 * other options:
   * clean the sources:  `platformio run -e generic -t clean`
   * erase flash: `platformio run -e generic -t erase`
+
 ### using the pre-compiled .bin files
 The pre-compiled files can be found on the [github page](https://github.com/tbnobody/OpenDTU) in the tab "Actions" and the sub menu "OpenDTU Build". Just choose the latest build from the master branch (blue font). You need to be logged in with your github account to download the files.
-Use a ESP32 flash tool of your choice and flash the .bin files to the right addresses:
+Use a ESP32 flash tool of your choice (see next chapter) and flash the `.bin` files to the right addresses:
 
 | Address  | File                   |
 | ---------| ---------------------- |
@@ -135,8 +136,31 @@ Use a ESP32 flash tool of your choice and flash the .bin files to the right addr
 | 0xe000   | boot_app0.bin          |
 | 0x10000  | opendtu-*.bin          |
 
-Make sure too uncheck the DoNotChgBin option. Otherwise you will maybe get errors like "invalid header".
+For further updates you can just use the web interface and upload the `opendtu-*.bin` file.
-For further upgraded you can just use the web interface and upload the opendtu-*.bin file.
+
+### Flash with esptool.py (Linux)
+```
+esptool.py --port /dev/ttyUSB0 --chip esp32 --before default_reset --after hard_reset \
+  write_flash --flash_mode dout --flash_freq 40m --flash_size detect \
+  0x1000 bootloader_dio_40m.bin \
+  0x8000 partitions.bin \
+  0xe000 boot_app0.bin \
+  0x10000 opendtu-generic.bin
+```
+
+### Flash with Espressif Flash Download Tool (Windows)
+
+[Download link](https://www.espressif.com/en/support/download/other-tools)
+
+- On startup, select Chip Type -> "ESP32" / WorkMode -> "Develop"
+- Prepare all settings (see picture). Make sure to uncheck the `DoNotChgBin` option. Otherwise you may get errors like "invalid header".
+- ![flash tool image](docs/esp32_flash_download_tool.png)
+- Press "Erase" button on screen. Look into the terminal window, you should see dots appear. Then press  the "Boot" button on the ESP32 board. Wait for "FINISH" to see if flashing/erasing is done.
+- To program, press "Start" on screen, then the "Boot" button.
+- When flashing is complete (FINISH appears) then press the Reset button on the ESP32 board (or powercycle ) to start the OpenDTU application.
+
+### Flash with ESP_Flasher (Windows)
+Users report that [ESP_Flasher](https://github.com/Jason2866/ESP_Flasher/releases/) is suitable for flashing OpenDTU on Windows.
 
 ## First configuration
 * After the initial flashing of the microcontroller, an Access Point called "OpenDTU-*" is opened. The default password is "openDTU42".
@@ -153,6 +177,8 @@ Navigate to Settings --> Firmware upgrade and press the browse button. Select th
 
 You'll find the firmware file (after a successfull build process) under `.pio/build/generic/firmware.bin`.
 
+If you downloaded a precompiled zip archive, unpack it and choose `opendtu-generic.bin`.
+
 After the successful upload, the OpenDTU immediately restarts into the new firmware.
 
 ## MQTT Topic Documentation