Add doc about external resources.

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diff --git a/doc/ExternalResources.md b/doc/ExternalResources.md
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+# External resources
+Since InfiniTime 1.11 apps and watchfaces can benefit from the external flash memory to store images and fonts. 
+This external memory is a lot bigger (4MB) than the internal memory that contains the firmware (512KB).
+
+This page describes how the resources are integrated in InfiniTime from a developer perspective. [This page](gettingStarted/updating-software.md) explains how to install and update the external resources using companion apps.
+
+## Resources generation
+
+Resources are generated at build time via the [CMake target `Generate  Resources`](https://github.com/InfiniTimeOrg/InfiniTime/blob/develop/src/resources/CMakeLists.txt#L19). 
+It runs 3 Python scripts that respectively convert the fonts to binary format, convert the images to binary format and package everything in a .zip file.
+
+The resulting file `infinitime-resources-x.y.z.zip` contains the images and fonts converted in binary `.bin` files and a JSON file `resources.json`. 
+
+Companion apps use this file to upload the files to the watch. 
+
+```
+{
+    "resources": [
+        {
+            "filename": "lv_font_dots_40.bin",
+            "path": "/fonts/lv_font_dots_40.bin"
+        }
+    ],
+    "obsolete_files": [
+        {
+            "path": "/example-of-obsolete-file.bin",
+            "since": "1.11.0"
+        }
+    ]
+}
+```
+
+The resource JSON file describes an array of resources and an array of obsolete files : 
+
+- `resources` : a resource is a file that must be flashed to the watch
+  - `filename`: name of the resources in the zip file.
+  - `path` : file path and name where the file must be flashed in the watch FS.
+
+- `obsolete_files` : files that are not needed anymore in the memory of the watch that can be deleted during the update procedure.
+  - `path` : path of the file in the watch FS
+  - `since` : version of InfiniTime that made this file obsolete.
+
+## Resources update procedure
+
+The update procedure is based on the [BLE FS API](BLEFS.md). The companion app simply write the binary files to the watch FS using information from the file `resources.json`.
+
+## Working with external resources in the code
+
+Load a picture from the external resources:
+
+```
+lv_obj_t* logo = lv_img_create(lv_scr_act(), nullptr);
+lv_img_set_src(logo, "F:/images/logo.bin");
+```
+
+Load a font from the external resources: you first need to check that the file actually exists. LVGL will crash when trying to open a font that doesn't exist.
+
+```
+lv_font_t* font_teko = nullptr;
+if (filesystem.FileOpen(&f, "/fonts/font.bin", LFS_O_RDONLY) >= 0) {
+    filesystem.FileClose(&f);
+    font_teko = lv_font_load("F:/fonts/font.bin");
+}
+
+if(font != nullptr) {
+    lv_obj_set_style_local_text_font(label, LV_LABEL_PART_MAIN, LV_STATE_DEFAULT, font);
+}
+
+```
+
diff --git a/doc/gettingStarted/itd-external-resources.png b/doc/gettingStarted/itd-external-resources.png
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diff --git a/doc/gettingStarted/updating-software.md b/doc/gettingStarted/updating-software.md
index d302607e..5c867023 100644
--- a/doc/gettingStarted/updating-software.md
+++ b/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)