Add doc about external resources.

doc/gettingStarted/updating-software.md

@@ -39,3 +39,35 @@ You can validate your updated firmware on InfiniTime >= 1.0 by following this si
 - Open settings by tapping the cogwheel on the bottom right
 - Swipe up until you find an entry named **Firmware** and tap on it
 - If the firmware is not validated yet, you can either validate the running firmware, or reset and revert to the previous firmware version
+
+# Updating resources
+
+Since InfiniTime 1.11 apps and watchfaces can take benefit of the external flash memory to store their pictures and fonts. 
+This external memory is a lot bigger (4MB) than the internal memory where the firmware is flashed (512KB). 
+Since those resources are not part of the firmware, they need to be flashed and updated separately. 
+
+Resources are packaged into a single .zip file named `infinitime-resources-x.y.z.zip` (where `x`, `y` and `z` are the version numbers of InfiniTime). 
+You can use the companion app of your choice to flash the resources.
+
+**Note : at the time of writing this page, [Amazfish](https://github.com/piggz/harbour-amazfish) and [ITD](https://gitea.arsenm.dev/Arsen6331/itd) have already integrated this functionality. Other companion apps will hopefully implement it soon!*
+
+## Amazfish
+Use the `Download file` functionality of Amazfish. 
+
+![Update resources with Amazfish - Download file](amazfish-external-resources-1.png)
+
+Amazfish automatically detects the file type (firmware or resources) and apply the corresponding flash procedure when you hit the button **Send file**.
+
+![Update resources with Amazfish](amazfish-external-resources-2.png)
+
+## ITD
+
+Run `itctl` with the `res` command:
+
+```
+itctl res load infinitime-resources-1.10.0.zip
+```
+
+Example:
+
+![Update resources using itctl](itd-external-resources.png)