Add another screenshot

macOS version/Asahi Linux Helper/README.md

@@ -11,23 +11,33 @@ Download: [Asahi Linux Helper.app.zip](https://github.com/rxhfcy/Asahi-Linux-Hel
 
 ## Random screenshots:
 
-![Description of Image 1](./misc/Menu_Screenshot.png)
-Main menu (click the menubar icon to open)
+![Main menu screenshot](./misc/Menu_Screenshot.png)
+
+Main menu (click "Restart in Linux..." from the menu to restart)
 
 ---
 
-![Description of Image 2](./misc/Restart_Dialog_Screenshot.png)
-Restart dialog (will polish later)
+![Restart dialog screenshot](./misc/Restart_Dialog_Screenshot.png)
+
+Restart dialog
 (dialogs use a popover "speech bubble" for technical reasons)
 
 ---
 
-![Description of Image 3](./misc/Preferences_Screenshot.png)
+![Preferences dialog screenshot](./misc/Preferences_Screenshot.png)
+
 Preferences dialog (temporary placeholder UI)
 (dialogs use a popover "speech bubble" for technical reasons)
 
 ---
 
+![Change default startup disk dialog](./misc/Change_Default_Screenshot.png)
+
+The app can also be used to change the default Startup Disk directly from the menu
+(for parity with the future Linux version)
+
+---
+
 ## GitHub project description / "about blurb" (early draft):
 "Asahi Linux Helper": Restart to Asahi Linux this time only (macOS menubar app)"
 (or say something more general?)