add the receovery mode description

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/tutorials/access_recovery.md

@@ -1,3 +1,113 @@
+The Recovery Mode is a special boot mode that can be entered when
+booting the **FunKey S** console.
+
+Its main purpose is to handle system updates, but it can also be used
+to recover the main system when it is no longer accessible, or to
+access some advanced test or developement features.
+
+## Enter the Recovery Mode
+
+There are 2 different ways to enter Recovery Mode:
+
+ - by holding both the <i class="funkey-start"></i> and the <i
+   class="funkey-fn"></i> keys at the same time while booting the
+   **FunKey S** console
+
+ - by typing the following command at the Shell prompt, either
+      using ssh over USB Ethernet or the serial console:
+
+```
+$ recovery_mode
+```
+
+While booting, the system will display the following logo instead of
+the familiar **FunKey S** logo:
+
+![Recovery Logo](/assets/images/Recovery_Logo.png){.lightbox}
+
+## Recovery mode commands
+
+The Recovery Mode automatically enables the USB network client upon
+startup, allowing ssh connection over the USB cable for debug
+purposes. For more information, please check the [SSH Connection over
+USB section][1].
+
+At the same time, the Recovery mode provides a simple text menu, with
+the current menu item displayed at the top of the screen.
+
+Navigation through the menu commands is using the <i
+class="funkey-up"></i> and <i class="funkey-down"></i> keys, and
+validation is using the <i class="funkey-A"></i> key.
+
+### INFO
+
+This command displays useful information regarding the **FunKey S**:
+
+ - the Recovery Mode version
+
+ - the rootfs (e.g. "Normal" Mode) version
+
+ - if connected to a host through USB network, the device IP address
+
+    ```
+    Recovery: 2.0.0
+	rootfs  : 2.0.0
+	IP addr : 192.168.137.20
+    ```
+	
+### USB MOUNT/UNMOUNT
+
+This toggle commands performs the same way as the FunKey menu command,
+except that no confirmation is asked.
+
+### USB CHECK
+
+This command performs a check of the FAT32 USB partition, and try to
+repair it if it finds some errors, such as when the partition was not
+cleanly unmounted from a host computer.
+
+### USB FORMAT
+
+This commands formats the FAT32 USB partition.
+
+!!! warning
+
+    This function will delete all the files on the USB partition, so be careful!
+
+### NETWORK ENABLE/DISABLE
+
+This toggle command will enable/disable the USB network upon next
+boot.
+
+### QR CODE
+
+This command will display a full screen QR Code containing the
+**FunKey S** serial number:
+
+![QR Code](/assets/images/qrcode.png){.lightbox}
+
+### FACTORY TEST ENABLE/DISABLE
+
+This toggle command will enable/disable the factory tests upon next
+boot.
+
+### FIRST BOOT ENABLE/DISABLE
+
+This toggle command will enable/disable the "first boot" sequence that:
+
+ - expands the rootfs "Normal" system partition
+
+ - create the swap partition
+
+ - create the USB shared partition
+
+ - install the free built-in games
+
+### EXIT RECOVERY
+
+This comand exits the Recovery Mode and reboots in Normal Mode.
+
+[1]: ../ssh_connection
 
 --8<--
 includes/glossary.md