CMake User Applications selection

Update documentation about building a new application and add instructions to add the app in CMake files.

doc/code/Apps.md

@@ -159,6 +159,24 @@ If your application is a **user** application, you don't need to add anything in
 everything will be automatically generated for you.
 The user application will also be automatically be added to the app launcher menu.
 
+Since the list of **user** application is generated by CMake, add a new `option` in the main [CMakeLists.txt file](../../CMakeLists.txt). The application will be built by default if the value is set to `True`. : 
+
+```cmake
+option(ENABLE_APP_MYAPP "Enable the MyApp application" True)
+```
+
+The default value can be overridden by passing an additional parameter to the command line of CMake : 
+
+```cmake
+$ cmake ... -DENABLE_APP_MYAPP=True ...
+```
+
+Then add your **user app** to the list of apps by calling `AddToListIfEnabled()` : 
+
+```cmake
+AddToListIfEnabled(USERAPP_TYPES_LIST ${ENABLE_APP_MYAPP} "Apps::MyApp")
+```
+
 You should now be able to [build](../buildAndProgram.md) the firmware
 and flash it to your PineTime. Yay!