Add documentation on the new plugin system (#179)

* Add documentation on the new plugin system (#178)

* Apply Code Review suggestions

---------

Co-authored-by: MatteoPologruto <109663225+MatteoPologruto@users.noreply.github.com>

docs/index.md

@@ -10,5 +10,10 @@ You have several options to install the latest version of the Arduino Firmware U
 
 Follow the [Usage guide] to see how to use the most common CLI commands.
 
+## Plugins
+
+Read the [plugins] section to learn more on the topic.
+
 [installation]: installation.md
 [usage guide]: usage.md
+[plugins]: plugins.md

docs/plugins.md

@@ -0,0 +1,101 @@
+# Plugins
+
+A new plugin-system has been implemented in the Arduino Firmware Uploader. A plugin is basically another
+executable/tool, specifically tailored for a board/family of boards, but with a well-defined user interface. Inside the
+plugin, there is all the business logic to support a board/family of boards. Every plugin implements the
+[interface](https://pkg.go.dev/github.com/arduino/fwuploader-plugin-helper#Plugin) contained in the
+[fwuploader-plugin-helper](https://github.com/arduino/fwuploader-plugin-helper) repository.
+
+The fwuploader is still responsible for downloading the files required for the plugin to work:
+
+- plugins/tools
+- certificates
+- firmware
+
+The new information is hosted in the
+[plugin_firmware_index.json](https://downloads.arduino.cc/arduino-fwuploader/boards/plugin_firmware_index.json), which
+is generated by `generator.py`.
+
+## uno-r4-wifi-plugin
+
+The source code can be found [here](https://github.com/arduino/uno-r4-wifi-fwuploader-plugin).
+
+### Known issues
+
+#### Espflash panic `UnknownModel`
+
+On some arm64 Linux distros, version 2.0.0 of [espflash](https://github.com/esp-rs/espflash/) might panic with the
+following error:
+
+```
+Error:   × Main thread panicked.
+  ├─▶ at espflash/src/interface.rs:70:33
+  ╰─▶ called `Result::unwrap()` on an `Err` value: UnknownModel
+  help: set the `RUST_BACKTRACE=1` environment variable to display a
+        backtrace.
+```
+
+#### The ESP32 module does not go into download mode
+
+On Linux, the UNO R4 WiFi must be plugged into a **USB hub** to make the flash process work. Otherwise, it won’t be able
+to reboot in download mode.
+
+```bash
+$ arduino-fwuploader firmware flash -b arduino:renesas_uno:unor4wifi -a /dev/ttyACM0 -v --log-level debug
+
+Done in 0.001 seconds
+Write 46588 bytes to flash (12 pages)
+[==============================] 100% (12/12 pages)
+Done in 3.106 seconds
+
+Waiting to flash the binary...
+time=2023-07-18T14:50:10.492+02:00 level=INFO msg="getting firmware version"
+time=2023-07-18T14:50:10.509+02:00 level=INFO msg="firmware version is > 0.1.0 using sketch"
+time=2023-07-18T14:50:10.511+02:00 level=INFO msg="check if serial port has changed"
+[2023-07-18T12:50:20Z INFO ] 🚀 A new version of espflash is available: v2.0.1
+[2023-07-18T12:50:20Z INFO ] Serial port: '/dev/ttyACM0'
+[2023-07-18T12:50:20Z INFO ] Connecting...
+[2023-07-18T12:50:20Z INFO ] Unable to connect, retrying with extra delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with default delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with extra delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with default delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with extra delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with default delay...
+[2023-07-18T12:50:21Z INFO ] Unable to connect, retrying with extra delay...
+Error: espflash::connection_failed
+
+  × Error while connecting to device
+  ╰─▶ Failed to connect to the device
+  help: Ensure that the device is connected and the reset and boot pins are
+        not being held down
+
+Error: exit status 1
+ERRO[0021] couldn't update firmware: exit status 3
+INFO[0021] Waiting 1 second before retrying...
+INFO[0022] Uploading firmware (try 2 of 9)
+time=2023-07-18T14:50:22.229+02:00 level=INFO msg=upload_command_sketch
+time=2023-07-18T14:50:22.230+02:00 level=INFO msg="sending serial reset"
+Error: reboot mode: upload commands sketch: setting DTR to OFF
+...
+```
+
+#### I flashed the certificates, but I am unable to reach the host
+
+The **whole certificate chain** is needed to make it work. Using
+[`-u` flags](commands/arduino-fwuploader_certificates_flash.md#options) (ex: `-u www.arduino.cc:443`) won’t work because
+it only downloads the root certificates. The solution is to use only the
+[`-f` flag](commands/arduino-fwuploader_certificates_flash.md#options) and provide a pem certificate containing the
+whole chain.
+
+#### My antivirus says that `espflash` is a threat
+
+The binary is not signed [#348](https://github.com/esp-rs/espflash/issues/348), and some antiviruses might complain. If
+still doubtful, https://github.com/esp-rs/espflash is open source, and it's possible to double-check the md5 hashes of
+the binary and the source code. For more information, you can follow
+[this](https://forum.arduino.cc/t/radio-module-firmware-version-0-2-0-is-now-available/1147361/11) forum thread.
+
+#### Not running on armv7
+
+At the moment, we are always downloading the armv6 binaries. Since they are dynamically linked, most likely they are not
+going to run on armv7. More infos on the topic:
+[here](https://developer.arm.com/documentation/ddi0419/c/Appendices/ARMv7-M-Differences/ARMv6-M-and-ARMv7-M-compatibility).