Update displaymanager.conf

Add an example to force Plasma5 desktop (commented out)

src/modules/displaymanager/displaymanager.conf

@@ -10,3 +10,9 @@ displaymanagers:
   - mdm
   - lxdm
   - kdm
+
+#Enable followed settings to force a destop environment in your displaymanager configuration file
+#default_desktop_environment:  
+#  - executable: "startkde"
+#    desktop_file: "plasma"
+