[README] Update readme file to add some hints.

README.md

@@ -28,6 +28,54 @@ Usage:  `./install.sh`  **[OPTIONS...]**
 
 > Bold version suggested use in `High resolution display` like 4k display with 200% scale!
 
+### New Icons
+
+To be able to change some icons :
+
+1. create a new one, ideally in svg format
+2. store it in place of : <*WhiteSur-icon-theme*>/src/apps/scalable/<*name_of_app*>
+3. reinstall icon theme, *for example* :
+
+```bash
+sudo ./install.sh -a -b
+```
+
+##### AppImages
+
+To apply **icons** for the AppImages :
+
+1. create a new icon if it does not exists already, ideally in svg format
+2. right click on the AppImage file
+3. click on the icon on the top left corner
+4. select the icon you want to have
+
+To have the icon and the app in the launcher / taskbar and so on ... :
+
+1. create a new icon if it does not exists already, ideally in svg format
+2. create a .desktop file in : /home/lmunier/.local/share/applications/ folder with the following specifications *as example, for SuperSlicer* :
+
+```bash
+[Desktop Entry]
+Encoding=UTF-8
+Type=Application
+Categories=Slicer;
+Name=SuperSlicer
+Comment=3D printing slicer.
+Exec=<path_to_your_appimage>
+Icon=/usr/share/icons/WhiteSur-dark/apps/scalable/super-slicer.svg
+StartupNotify=false
+Terminal=false
+Hidden=false
+StartupWMClass=SuperSlicer
+```
+
+3. StartupWMClass specification allows the OS to have the right icon when the program is running. If you do not know what to put :
+   1. open the program
+   2. do : Alt + F2
+   3. run command : lg
+   4. click on *"Windows"* tab
+   5. check the *"WMClass"* name and set it for *"StartupWMClass"*
+
 ## Requirement
 You can use this with: